Interestingly, former Clinton administration officials have said they knew North Korea was cheating on the uranium enrichment front dating back to 1998 and planned to use that intelligence as leverage to keep the Agreed Framework in place and the plutonium under lock and key. Other Clinton administration officials will also concede that they never thought they would have to build the light-water reactors because they assumed, wrongly, that the regime would collapse before the reactors would be built.
North Korea was cheating because they could. Because they had already received concessions, funds, equipment. Bush stopped adhering to the terms of the deal - which was to give them even more equipment, even more money.
Of course, North Korea used that money (extracted, by the way, from South Korea, the United States, AND Japan) to build their current nuclear arsenal.