This makes no sense to me. $15,000,000 would have bought a lot of ads in the various swing states. It's not like he's going to need the money for his next senate run since he's the incumbent democrat from Massachusetts. He'll win that race regardless. He wouldn't even need it for potential legal challenges if the election had been a squeeker. George Soros would have been willing to cough up almost any amount for lawyers if they had needed them. His wife's worth about $1,000,000,000 so a few million in campaign funds isn't going to affect his personal finances much. Only a moron would try to save some of the money from a close race for a long shot campaign four years from now and I don't think he is a moron.
So the question is, "why the hell didn't he spend it?" Like Saddam's missing WMD's I think there is more here than meets the eye.