Because they think the nomination will still be in doubt come March. A month ago, it looked like that might be true, but not now. Hillmentum has carried the day.
Hillmentum has ended up being a bit more uphill than I thout it would be, but only Berniebots deny the obvious. Even were there no superdelegates, Hillary would have a majority of the delegates when the DNC convenes in Philadelphia. And just to remind you, I voted for Bernie, and still would if the February primary were voided and I had to vote again.
