Because LOST was six seasons long. That's 121 episodes, or eighty-four hours and forty-two minutes of solid television. Think about that. If you decided to do nothing but watch LOST for eight hours a day, five days a week, it would take you more than two weeks to get through the whole thing. That's a lot of build-up to get to a one-hour payoff.
And of course, that's not even the full story. Because let's face it, the people who were really into the LOST finale had been watching the show since the beginning. Sure, it wasn't a continuous time commitment, but the really big LOST fans put six years of anticipation into the ending of the series. Six years of discussing, dissecting, examining and cross-examining in the firm and fervent belief that the series would reward that degree of enthusiasm. (Ironically, Penny Arcade got it more or less right from the start.)
And what did they get? A fairy-tale ending. The Island is a big magic power source guarded by an immortal, who is looking for a replacement. The guardian has an evil shapeshifting brother who covets the power and hates the guardian. There's a big fight. Good wins, but not without cost. It's a decent enough ending, fairly common and no better or worse than the people writing it, but the people who put six years and eighty-four hours of emotional investment into the show were never going to be satisfied with that.
Honestly? They were never going to be satisfied, period. You can't come up with an ending good enough to justify 121 episodes of build-up. That's the danger of series built around big mysteries, from LOST to BSG to Twin Peaks. The answer to "Who killed Laura Palmer?" is always just going to be, "Some person," and the longer you wait before revealing the answer, the more irritated people are going to be that they waited that long. Building a series around one big mystery is risky for that very reason. Sometimes it's better to have a show where every episode can stand alone. The pay-off may not be as big, but at least it won't fall flat.