Why Didn't Obama Go to Congress Before Attacking Libya?

Because he didn't have time with his vacation pending.  Ace explains:

It's quite simple. The President didn't go to Congress because he never thought he'd need to go to Congress. Obama spent three weeks dithering and then almost a full week telegraphing his intent not to intervene. But when the time came to announce his decision, he flinched and made a last-second gut decision to go to war.

The decision to commit the United States to war wasn't out of any sudden change of heart about the value of Libyan lives. Nor did the President suddenly discover U.S. national interests in North Africa. He did it because he was getting internationally embarrassed by the French and by Secretary Clinton. He did it because he was looking bad and after three and a half weeks of polling his numbers were looking worse.

So, having failed to make any effort at all to reach out to Congress on the issue because he never expected that he would have to and with his Brazil vacation imminent, there simply wasn't any time left to get Congressional authorization. Yes, he could have gotten it, in the sense that I'm absolutely sure the votes are there. But it would have taken a few more days and not even the MBM could pretend that he was "leading" on the Libya issue at that point.

The parade was passing by so Obama ran to the front to pretend he'd been leading it all along. Perhaps he forgot about the Nobel Peace Prize he won after being president for two weeks.

Doug Mataconis adds this via Twitter:

President Obama and his supporters seem to think that the UN's approval is all they need to justify this action. Congress is now irrelevant

That certainly would be in keeping with his "President of the World" mentality. By the way, Ace thinks that going to war without congressional approval constitutes a "high crime" and is worthy of impeachment. I doubt it will ever go that far unless Libya becomes a complete disaster.