There are a couple of reasons.
First, it’s rare that politicians are still in office when the unintended consequences of their grand plans begin to come to fruition.
Second, a key problem I have with government is the feedback. Government activities are intention-driven, rather than results-driven. As Milton Friedman once said (HT: Cafe Hayek):
Well, there are fewer limits on what you can promise than on what you can deliver.
But, since Obamacare touches everybody, I think results may matter more.