First of all, the credit crunch happened across the globe. Labour was not responsible for it. While the Conservatives officially state that it was the fault of Labour and that Labour had overspent, they had made a commitment to match the spending of Labour, and after the Credit Crunch started to cause issues, they came up with the Redwood report. And if this was in place, our nation would have been more exposed and fared worse. In fact, there was one politician who was aware of the risks that we faced, and I remember his warnings in the lead up to the crunch, and he was not a Tory (though he was considered an honorary one between 2010 and 2015).
And while the Conservatives would not admit this officially, they know that the economic crisis was not the fault of Labour. But it does not stop them repeating the lie.