PDQ, I asked the same question about Chalie Gibson not interviewing Joe Biden -- but from the opposite point of view.   
How come Sarah Palin gets 2 or more nights in prime time, plus Nightline, to answer questions from a network anchor/interviewer and demonstrate his knowledge, judgment, views, and readiness to be VP or even step in as President if need be?

I'll bet Joe would jump at the chance.

G

PS,
Yes, I know Sarah Palin is a "new face." But most of the country doesn't know Joe Biden all that well, either -- any more than they know most of the 100 US Senators.