Telltale have updated their Back To The Future website at last, with Character and Episode pages now up.
On the character pages you can look at concept art, read a short bio (for character and actor), and even listen to voice clips. Characters currently available are Marty, Doc, George McFly, and for the first time, Biff! Sadly Tannen(s) will not be voiced by the iconic Tom Wilson, instead Telltale veteran with a great name Kid Beyond will be doing the voice. There's at least one other character still to be revealed - we haven't had a woman yet, so Jennifer, Clara or Lorraine perhaps?
Also excitingly on the site are short plot synopses for each of the five episodes! I won't post them here in case anyone wants to remain unspoiled (plus it's a flash site so I can't just copy and paste them like the lazy git I am), but episodes 2 and 4 do contain concept art of two other characters - mafia gang boss Kid Tannen and a mysterious woman (Clara?).
Go see! Allons-y! (sorry, wrong time traveller)
UPDATE: There's also a new interview up at GameTrailers focusing on story and gameplay! Haven't seen it myself yet, but it seems like it detail just how the game is going to play, which is something a lot of people (myself included) have been wondering.
Source: Telltale
My guess would be he just doesn't want to be associated with that role anymore.
I wonder if they even approached him to reprise the role.
Maybe he wants to distance him away from the series, although I can't see a reason why he would want to do that.
However I did notice his lack of presence in 25th anniversary celebrations. So this could be related.