Some are weak programs, but then again other weak programs did fill this year. Lots is location, ie Penn State. Excellent program, excellent residents, huge cases, but the rural location turns a lot of people off. UMass is another one, Worcester isn't so happening but they have an excellent med school there. It isn't the best program in the world, but it is a decent university program that actually has at least one big name professor and they do some research. Other programs, NYU, Cornell, and Rush probably overestimated how popular they were and didn't rank enough people even though they got more than enough apps. Both are excellent programs by the way. Still others lack name, ie Baystate. Don't look at the list and assume that unfilled programs are weak. Look for trends over the last several years and you will see that some programs never really fill at all.