First, while the democrats took control of congress after the 2006 midterm elections, they took control with a less than confidant majority of votes. In the house, the Republicans lost 3.6% of the popular vote--not exactly a landslide. Impeachment polls aren't all that common; Googling bush impeach poll doesn't yield any sites of recent mainstream media polls on the first page (contrast this to approval ratings). After the drawn out non-binding resolution to not support the action in Iraq, the current congress shows that they lack the political dedication and courage (balls, cajones, if you will) to carry through with meaningful action. Between their lack of gravitas and limited support, the second to last thing they will do is impeach Bush (the last being removing him from office).
The second reason is half comedic. Bush is a political paradox. The same people who are so quick to call him an idiot are the same who claim he's the mastermind of everything going wrong. Only Bush can not like black people, have black cabinet members, provide little immediate federal response to the attack on the WTC and be praised, provide more support to New Orleans and be cursed. When praising Bush it appropriate, he is easy to praise. When things go wrong, he's easy to blame. When jokes are needed, Bush is an idiot. When conspiracies and scandals emerge, Bush is the villain behind them. The Democrats can't impeach Bush because they could no longer place political blame so well on a single person. Back to the first reason, while Bush's ideas might not be the best, the Democrats have even less confidence in their ideas, and provided Bush is in power, when something doesn't go well, he can take the blame.