A variety of reasons. From the Democratic side, Obama could get most of what he wanted with the fiscal cliff by waiting, and his position was overwhelmingly popular. This gave him the opportunity to publically seek compromise while negotiating relatively aggressively. The Democratic position on the sequester is less popular, and current law doesn't lead to it, so it's harder to push -- instead of saying "the GOP is holding up tax cuts for 98% of Americans," it has to be "the GOP is forcing spending cuts on the whole government." The problem is that people like spending cuts in the abstract, so that argument doesn't impress much until after the cuts hit.
From the Republican side, well, as always, the Republican-controlled House can't actually come together and pass a bill to avert the sequester. That's why they keep talking about the bill they passed in the LAST Congress as though it's a viable alternative, though it's actually legally dead. Any sequester-averting bill is going to pass with mostly Democratic votes -- but Boehner can't bring up any such bill until he's forced. After the sequester hits, if the Senate passes something, he may be able to bring it up then, just as he did during the fiscal cliff. But he already promised not to negotiate, remember? Without negotiation, and with a caucus that can't actually pass anything, there's nothing for him to do except wait.
Basically, as with the fiscal cliff, both sides are afraid of the political backlash from acting to prevent spending cuts, so nothing will happen until public opinion turns against the spending cuts. Hopefully that will happen when the spending cuts actually take place and people realize they like having parks and police forces.