Thomas has played 12 games to Fernandez's 6 (7 counting tonight), and Fernandez is making a solid chunk of change ($4.33 million), plus has had significant success in the past with Minnesota and is coming off an injury, so before the Bruins turn to Rask (and so they don't completely tire out Tim Thomas) they need to make sure Fernandez gets a fair shot at returning to the form he had in Minnesota before the injury last year.
That being said, I hate Fernandez and want Thomas/Rask as the goaltending tandem, but where do you put that 4.3 mil?? No one's going to take that on with him playing like this, so they have to just wait and hope he returns to form. God knows they could use the extra dough to make an additional acquisition and take a serious run at the cup.