Because their ideas don’t work. Paul Krugman spells it out:
Four years ago, right-wing economic analysts insisted that deficit spending would destroy jobs, because government borrowing would divert funds that would otherwise have gone into business investment, and also insisted that this borrowing would send interest rates soaring. The right thing, they claimed, was to balance the budget, even in a depressed economy.
Now, this argument was obviously fallacious from the beginning. As people like me tried to point out, the whole reason our economy was depressed was that businesses weren’t willing to invest as much as consumers were trying to save. So government borrowing would not, in fact, drive up interest rates — and trying to balance the budget would simply deepen the depression.
Sure enough, interest rates, far from soaring, are at historic lows — and countries that slashed spending have also seen sharp job losses. You rarely get this clear a test of competing economic ideas, and the right’s ideas failed.