Many reasons, but cut to the chase:
[...] The voices of protest bump up against the stubborn reality of divided government. It’s possible, but not probable, that Democrats will get the 24 seats they need to regain control of the House, but there’s a stronger possibility the Republicans will take the Senate; they only need four seats. “If they have the presidency, then the Tea Party is running the country," says Schneider, and Democrats "can’t take any chance that will happen."
There you have it.