In a nutshell, because it worked:

Asked about the S&P assessment, 71 percent of Americans called it a fair one. On the blame front, 36 percent say the GOP is culpable for the downgrade, 31 percent blame Obama and his fellow Democrats and 22 percent say it’s both sides equally.

So first the House Republicans held the debt ceiling hostage. It's utterly clear that this which caused S&P to downgrade U.S. debt:

A top official at rating firm Standard & Poor's said Friday the company's decision to downgrade U.S. government debt for the first time in 70 years was due in part to Washington's political paralysis surrounding raising the debt ceiling.
The "conclusion was pretty much motivated by all of the debate about the raising of the debt ceiling," John Chambers, chairman of S&P's sovereign ratings committee, said in an interview. "It involved a level of brinksmanship greater than what we had expected earlier in the year."

The reasons for the downgrade may be shaky, but that's neither here nor there. S&P got freaked out by the hostage drama and decided to downgrade the debt.

But, of course, Republicans aren't going to say that they caused the downgrade. They're going to blame the Democrats, as parties do. And the news media isn't going to take sides in this. We're going to get a shouting match. Indeed, even the liberal Daily Show presented it as a childish shouting match in which neither side was right:

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End result: Republicans get concessions, and when the consequences for their decision hurt everyone, the blame is spread equally. If you can use hostage tactics and walk away with both a ransom and everybody blaming you and the ransom-payer equally, why not do it again?