your probably figuring that the liberal would fire up the base, but it wouldn't work. if the VP is two different from the nominee it creates conflict, because you have the one guy saying we should do this, and the other guy saying that's wrong. they woudl inadvertantly make each other look bad.
for example the only thing Bush and Cheney disagreed on was the constitutional ammendmant to ban gay marriage, but all Cheney would say about it was "I used to support it, now I don't, but Bush supports it, he's my president and I support him"
they can't be saying that about every issue because they would come off as opponents rather than partners.