Because you forget ... that to let the tax cuts for the rich expire, he'd also have to have let the tax cuts for the middle class expire too.
And if he did that, he's a one term President.
The left would not be praising him much for letting those top end cuts expire ... much like they don't really think DADT was a big deal, saving the auto industry, health care reform, financial reform ... the response from the left would be tepid on this point ... perhaps even negative because, again, those middle class tax cuts would have gone away ... some here on DU would be complaining that he allowed that to happen. Used it to call him weak.
Meanwhile, if he did let them all expire, the media would be crucifying Obama for raising the taxes of the middle class during the west economic crisis since the great depression.