The stated reason was this:
We lowered our long-term rating on the U.S. because we believe that the prolonged controversy over raising the statutory debt ceiling and the related fiscal policy debate indicate that further near-term progress containing the growth in public spending, especially on entitlements, or on reaching an agreement on raising revenues is less likely than we previously assumed and will remain a contentious and fitful process. We also believe that the fiscal consolidation plan that Congress and the Administration agreed to this week falls short of the amount that we believe is necessary to stabilize the general government debt burden by the middle of the decade.
The problem isn't that tax revenue wasn't raised, nor spending cut. The problem was that what should have been a relatively simple, procedural vote was turned into a political quagmire. The problem was that someone decided that economic stability should be used to gain political leverage.
Well, it worked.
And a couple days after these nitwits were gloating about how happy they were with the deal, S&P spoke up with a message that needed to be said, but that nobody wanted to hear. A message that the American people have been repeating since this crap started, as evidenced by the poll figures on the question "who is responsible for the debt ceiling crisis?" placing the blame squarely on conservative shoulders.
That message was this: We don't want a repeat of this bull****. We don't want long-term economic stability used as a tool to leverage short-term political gains. We didn't want it in April, when this issue came over the horizon. We didn't want it in July, when it finally came to a head and forced Congress to act.
Teapublicans didn't want to listen to the American people when they sent this message. Teapublicans blatantly ignored that message when it was delivered by the people. They didn't listen to the ratings agencies when they reported on July 25th that Boehner's plans would lead to a downgrade.
John Boehner got 98% of what he wanted. Mitch McConnell vowed a repeat of the same tactic in the future:
Well, Boehner, McConnell, Republicans, and especially Tea Party representatives, now that the Corporate overlords have spoken, showing their "approval" of your negotiating tactic with the second largest stock market drop in history and repeating the same message delivered to you by the American people over the past 4 months, are you finally going to listen? Or will it take recall elections to make it happen?