Obama needs the Catholic vote and Sibelius would really hurt him. She has been banned from receiving communion by her bishop for her abortion positions.
I don’t think the Dems want to go though all of that again.
A lot of undecided voters don't really care about abortion when it comes to the presidency. Bush had no effect on the issue so it seems a moot point. Maybe that's what Obama is counting on, and again, with Sibelius, Obama presents a clear difference to McCain.
Remember, the Dems are presenting McCain as the establishment candidate, Bush II. They can't hold that line with by picking an established insider.