The reason is as straightforward as the character's motivation: Sauron is a one-dimensional non-character, a plot device. His ring is more interesting than he is. He has spent ages trying to do this one thing, to rule everyone, with apparently no good plan for doing so and no reason to do so other than "I'm Sauron." From being a servant of Morgoth until the end of the series, all he does is try and try and try to be a jerk of the highest order.
We never really find out why; the fact that he is thwarted constantly is about all the motivation he is given. He tries to conquer the will of all, his overly-elaborate plan fails because of his apparent arrogance and misunderstanding of human (and elven and dwarven) nature, he fades and then starts revving up to do it all over again. He is the meanest, most thick-headed one-trick pony of all time.
He is there to power the central conflict of the series, true, but one of the failings of Tolkien's saga is that there is a fairly rich cast of characters on one side, and on the other are teeming hordes, a sniveler, a guy who can't commit, and a mustachio-twirling mega-villain that for unfathomable reasons the rest follow. I resent Sauron because I want there to be more complexity and motivation on the other side. The contrast is too stark, too essentialized, and most of that comes from the figure of Sauron, who is evil incarnate, but a very stereotyped, unexplicated evil that has nothing to substantiate it but the biggest chip on one's shoulder in the history of the Universe.
(Pardon the lateness; laid down with child last night and dozed off)