Because;Hang on a second, our credit rating WAS destroyed.
a) We were spending 150% of tax revenues, three years later, and had no plan to balance the books.
b) As asset prices kept collapsing, the losses on the balance sheets of the banks got bigger, wiping out more risk capital, leaving a bigger hole for the state to fill. You will recall that we borrowed €2.5 Billion from the Troika to cover the estimated hole as at the end of 2010, but we also had access to an additional €15 Billion from the Troika if there were any more losses that needed to be replaced.
c) The markets weren't keen on European Sovereign Bonds at that time because Angela Merkel was talking about making them significantly more risky to buy.
Any one of those problems, on their own, would have caused problems, but the three of them together was a perfect storm (of our own making - see FFailure ponzi scheme economics circa 2002-2007)