This is a good thread to answer your question, but to sum up:
Mainly because until the events of Diablo 3, demons had difficulty entering Sanctuary, and their manifestations were not capable of drawing upon their full power, and because the protagonist of each game gains experience and equipment over the course of their story (so an even challenge at the beginning is "a few skeletons", and at the end, it's "Diablo himself").
Some specifics: in Diablo, the final boss is not Diablo himself, but Diablo possessing the son of King Leoric; the canonical hero, Aiden, is himself a well-trained warrior, with capabilities and prowess far beyond that of your average Tristram peasant. At the end, Aiden attempts to contain Diablo within himself by smashing Diablo's Soulstone into his head; this very ill-advised tactic results in him becoming the Dark Wanderer of Diablo II, so it's not like humans are across-the-board more powerful than demons.
In Diablo II, the Evils are still bound by their Soulstones, and Baal specifically possesses another human body (that of the nephalem Tal Rasha), again limiting his full power. You do fight Diablo at his full power in D2, but only after journeying far and wide and -- again -- gaining a great deal of experience, equipment, and abjurations.
Finally, the fact that the Diablo III protagonist is a nephalem is very significant. Unlike their predecessors, they theoretically have access to the untested powers and abilities of the original nephalem, the combined offspring of angels and demons. This is so because at the end of Diablo II, the Worldstone, originally created to limit humans' power by agreement between the High Heavens and the Burning Hells, is destroyed, enabling the nephalem to once more return to the world of Sanctuary. As for why, if this is the case, the nephalem hero can't simply mop the floor with the Act IV boss: at the end of Diablo III, Diablo is no longer one being but combines the powers of all his brethren with his own to form the Prime Evil, a being which would have easily overwhelmed the heroes of Diablo 1 and 2.