The recession. The loss of white-collar jobs caused by the recession resulted in white-collar voters becoming Democrats to this day, and the appointment of Clarence Thomas alienated socially liberal voters who had been Republicans for economic reasons, and him breaking his tax pledge depressed voter turnout. Clinton was also simply more charismatic than HW Bush. Clinton talked about how he could relate voters and had been through similar things.
This, but I would also add Conservative alienation due to Souter's appointment. There was also Pat Buchanan's primary run from the right, voter fatigue after 12 years of Republican rule, and the fact that Bush ran a terrible campaign in 1992. He waited too long to start campaigning thinking the Gulf War alone would give him a victory, and he offered nothing with regards to what he wanted to do in a 2nd term. Bush also wasn't one to brag and thus didn't talk about the many foreign policy successes of his either.
And no, Perot DIDN'T cost Bush the election!!!!!!!!