This is why.
Obama is still president, and can veto anything they try to pass. Despite the huge scope of their House victory, the Democrats still control the Senate, albeit by a small margin. And Republicans are not popular with the American people; they were elected as an alternative to something even worse.
So, what can they do? They can’t pass legislation over a presidential veto unless they can get enough Democrats on board to help them override it. This is highly unlikely—unless a great many Democrats feel the hot breath of their own imminent departure from office in 2012 if they don’t get with the program. That’s also highly unlikely; many of the remaining Democrats in the Senate don’t even come up for re-election until 2014 or even 2016, and are immune to such a consideration, or are from true-blue states that would consider voting with Republicans to be a betrayal.
So this just may be the most sober tsunami election in history. Republicans may be able to hold off Obamacare through defunding; it’s unclear how that would work. What’s more, since they were elected to stop things rather than accomplish things, they can keep reminding people of what might have been passed (cap and trade, for example) if they hadn’t been elected. They can pass bills in the House and force Democrats to choose to vote yea or nay, while also getting Obama to go on record as vetoing some popular legislation. It’s possible Republicans can even drag enough Democrats on board to pass a few bipartisan (remember that?) initiatives such as tort reform, which Obama may hold off from vetoing, and thus Republicans could actually point to some accomplishments.
If people are savvy enough to understand the limitations of the current situation, they might forgive Republicans for not accomplishing much, as long as they keep their word and try to stop the most offensive moves of the previous Congress. They would also earn points by being sober and serious and focused on the task at hand.
It doesn’t seem as though expectations of those who voted for them are very high; my guess is that most people may understand how hard stopping or reversing the Obama-Pelosi-Reid legislative train will be.
[NOTE: I suggest that the Republicans not spend time attempting to investigate and subpoena Obama or his deputies. i don't think the American people would appreciate more attacks and blaming; it will perceived as more divisive politics as usual, and they've had enough of that. The Republicans need to be the people's servants, and stick to the reason they were elected, which is to stop the programs they don't like, and attempt to get our fiscal house more in order.]
[ADDENDUM: Oh, and here's a reminder that in 2012, there are 21 Democratic seats up for re-election vs. 10 Republican seats. That's a big big difference. And quite a few of those Democratic seats are in states that are not so strongly liberal as this year's crop.]