Maybe because I read Three Hearts & Three Lions at an early age along with many of the Arthurian romances plus C.L. Lewis's Lion, The Witch, & The Wardrobe. Couple this with a huge swath of Robert Howard's Conan, classic era Marvel comic books & other sundry Sword & Sorcery paperbacks. The one constant in many of these sources was the appearance of the troll. The troll as a monster was well established in OD&D thanks to Paul Anderson's Three Hearts & Three Lions.
"In Poul Anderson's Three Hearts and Three Lions, trolls are depicted as eight-feet tall creatures, man-eating creatures which are almost impossible to kill, since their bodies immediately re-knit and repair any damage, however severe. Even beheading a troll is of no avail - he would simply pick up his head and place it back on his neck and within a moment be "as good as new". A troll's only vulnerabilty is to fire - the only way to kill one is to cut him to pieces and burn each and every one of them before they had a chance to re-knit."
That quote is from the Wiki entry on trolls in popular culture we get a pretty consistent view of them in popular culture from Three Billy Goats Gruff, right on through the comedy series Ugly Americans.
Trolls are ugly, very dangerous, & seem to be classified with giants from Scandinavian legend & mythology.
"A troll is a class of being in Norse mythology and Scandinavian folklore. In Old Norse sources, beings described as trolls dwell in isolated rocks, mountains, or caves, live together in small family units, and are rarely helpful to human beings.
Later, in Scandinavian folklore, trolls became beings in their own right, where they live far from human habitation, are not Christianized, and are considered dangerous to human beings. Depending on the source, their appearance varies greatly; trolls may be ugly and slow-witted, or look and behave exactly like human beings, with no particularly grotesque characteristic about them."
So how does this relate to the popular monster of Dungeons & Dragons? I've noticed, there are tons of species of troll based around various elements, geologies, & planar conditions; "In Dungeons & Dragons, the Trolls are depicted as tall skinny humanoids with long noses, rubbery skin, a regenerating ability, and a weakness to fire. Other types of Trolls include the Black Trolls, Blood Trolls, Cave Trolls, Crystalline Trolls, Desert Trolls, Fell Trolls, Fire Trolls, Forest Trolls, Giant Trolls (which is a crossbreed between a troll and a Hill Giant), Giant Two-Headed Trolls (which is a crossbreed between a troll and an Ettin), Gray Trolls, Ice Trolls, Mountain Trolls, Rock Trolls, Scrags, Slime Trolls, Spirit Trolls, Stone Trolls, Tree Trolls, War Trolls, and Wasteland Trolls. In addition, they keep the canine-like Trollhounds as pets in their tribes."
Trolls have three things going for them,the ability to adapt to any conditions they're put in, the ability to regenerate from any damage, and an almost artificial violence & ferocity toward others.
Many of the giants & trolls of Arthurian legend are far closer to the classic Giant Troll from the Fiend Folio Advanced Dungeons & Dragons first edition. A giant troll is a cross between a troll and a hill giant, resulting in a troll over 10 feet tall. Once again we've got a species of monster whose close but not quite human living & mating with other monsters to produce another species. This can't be simply random chance, I'll postulate that perhaps the classic troll species is a product of an ancient experiment going wrong. Sure I'm not the first to say this but there's a bit more to it. The mythological weakness to 'sunlight turning trolls to stone' & fire is troubling but we'll get to that.
Erol Otus from the AD&D Fiend Folio a classic piece of artwork.
I think that the answer for the origin point for the modern troll species might actually be found in the original Three Hearts & Three Lions book by Paul Anderson.
"This world is divided between the forces of Chaos, inhabiting the "Middle World" (which includes Faerie), and the forces of Law based in the human world, which is in turn divided between the Holy Roman Empire and the Saracens."
This arrangement of the planes is definitely artificial as we see reading through the novel. But its important to note that Faerie plays a central role in the Paul Anderson novel in terms of both secondary characters & the antagonist's forces. The forces of Faerie are fickle at the best of times towards mankind & downright malevolent at the worst of times.
Looking into the legendary background of classic mythological Faerie nature spirits & Elves I came across tid bit on Wiki ;"Another belief is that the fairies were an intelligent species, distinct from humans and angels. In alchemy in particular they were regarded as elementals, such as gnomes and sylphs, as described by Paracelsus. This is uncommon in folklore, but accounts describing the fairies as "spirits of the air" have been found. The belief in their angelic nature was common in Theosophist circles"
If we look deep into classic mythology primal Chaos was one of the first things to exist; "In Greek mythology, Chaos (Greek: Χάος, "Chasm") was the first thing to exist: "at first Chaos came to be" (or was) "but next" (possibly out of Chaos) came Gaia, Tartarus, and Eros (elsewhere the son of Aphrodite). Unambiguously born "from Chaos" were Erebus (Darkness) and Nyx (Night)" its also a powerful force of occult energy. The troll species is always associated with giants, or humanoid kind along with humans. We even find this connection in J.R. Tolkien's Lord of the Rings series; "In The Lord of the Rings franchise, there were different types of trolls ranging from Stone Trolls (which turn to stone when exposed to sunlight), Two-Headed Trolls, Hill Trolls, Cave Trolls, Mountain Trolls, Snow Trolls, and the Olog-Hai (the type of Trolls that can withstand sunlight)."
Once again we've got that elemental adaption connection, the artificial species connection, & the presence of spirits creating new possibly human connected species powered by primal chaos; "The Olog-hai were a fierce race of Trolls. They only spoke Sauron's Black Speech, and it is possible they only existed because Sauron's evil will empowered them. They were bred by Sauron from an unknown stock, though it's suggested that they could be corruptions of some primitive human types." There are lots of varying opinions when it comes to the Olog-hai.
As a dungeon master I stay away from Tolkien because of these varying opinions & because gamers feel so strongly about the Middle Earth material.
Trolls make excellent Sword & Sorcery monsters especially when their origin crosses human, giant kind, & something else. We would need a race of highly advanced masters of the biological & engineering arts who had been here since Earth's origin & were capable of planar creations & demi planes (Faerie). Oh wait we already have all of those in HP Lovecraft's at the Mountains of Madness Elder Things.
Toward the end of their reign on Earth when mankind was first emerging the Elder Things were looking for a replacement for the rebellious shoggoths. A breed of easily adaptable workers, army,etc. They began to experiment & some of their first creations could not exist in sunlight & were vulnerable to flame.
The latter half of the Elder Thing's empire was fading fast & it was already too late. 'The Shoggoth Rebellion' destroyed millions of years research. The first & secondary Ice Age Floods did the rest. There are qualities that trolls have in common with shoggoths from regeneration to a pastiche for adaptability.
The laboratories in Troughburn, in Northumberland, England, were erased from existence originally Trolhop, Norse for "troll valley" is now only a dim racial memory of a place far to horrid to have existed. The troll species is regarded today as toxic waste spread across planes. How did this come to pass? We & other species found the nearly immortal proto trolls & wizards finished the work of the Elder Things. The Elder Things' city is identified with the Plateau of Leng which has been the site for pilgrimages by necromancers, wizards, & occultists for centuries. It has been speculated among certain circles of gamers over the years that the knowledge of the creation of trolls was spread by Nyarlathotep through "The High Priest Not to Be Described. "
I have yet to run one game that does not feature trolls so players beware for they are a hidden menace!