Because it was won by people who don't like the GOP either. This was dems voting for a smaller government.
Also... This was Dems voting against those who make their party look worse than it really is. My prediction is that Reid and Pelosi are on their last term.
The big thing about MA is that there are twice as many Independent voters as there are D's and R's combined! Independents are growing nationwide as people grow tired of the Democrat/Republican bickering.