They hate him because he is not a democrat. Plus, he supports the war in Iraq (at least this week, maybe he'll "evolve" on that as well).
Of course, this kind of destroys the argument that Rudy appeals to liberals and independents and therefore can beat Hillary.
And it's funny to attribute any rationality to the democrats actions, other than that they want a person in the white house with a "D" by their name, no matter what they believe.
Sounds eerily similiar to the recent hosts of republicrats - 'cept for the letter of choice.