The economy was always a house of cards waiting to fall. It was all based on a 'good feeling' with little substance. Soon as American banks started questioning whether these mortgages were worth the paper they were written on, it collapsed. Labour bought in to the grow-and-grow myth, so they take the blame for it collapsing (quite rightly). Tories wouldn't have done much different, which is why I get more annoyed that votes switch back to another failed party with similar policies.
Same as the expenses scandal. Absolutely disgusting and the government deserves to fall. But do we really have to replace them with a party that's also damaged?
Having said that, I want Brown to stay for a little while, so we can really get rid of those that deserve to in the wake of the expenses. They don't leave? Then in a Labour ward, get an independent Labourite in. Same as in Tory wards. Then maybe get a complete change of government, not just a 'different' party.
I think another reason was, like Labour in the 80s, the Tories were such a mess, that they were completely unelectable. So much infighting that you couldn't expect a Tory Prime Minister to last much more than a year. It's taken a decade but now they seem vaguely behind their leader (who represents the Tories almost as much as Blair did Labour, i.e. not too much)