It’s all about the Electoral College.
Here’s Obama vs. McCain based on current polling:

Most significant here is the fact that states like Pennsylvania (21 Electoral Votes) and Ohio (20 Electoral Votes), both of which Obama lost to Clinton, are too close to call. If those two states moved into the McCain column, he’d have 281 Electoral Votes and he’d be the next President of the United States.
And here’s the Clinton vs. McCain matchup:

The biggest difference here is that Pennsylvania, Ohio, and Florida are all leanding toward Clinton. That’s a total of 87 Electoral Votes right there.
The electability argument isn’t as absurd as some of Obama’s supporters might think, but Clinton is running out of time to make it.

