First off, Red Sox - DRays IS a rivalry. But the gist of it is that BC is a relatively small school in a suburb, and it doesn't connect with the majority of people in town. This is a city with many colleges and universities. In that sense, it's unlike most markets. Northeastern, BC, BU, MIT, Harvard, Tufts, Suffolk, UMass-Boston, Emerson, etc. I bet BC-PSU would do decently--better than BC-OSU, but there is simply little interest in college sports. That goes for basketball and hockey as well (outside of the Beanpot, which is the only event that gets people interested in college sports in Boston). But that's only 2 days of the year.
The fans not only don't relate to the opposition, they don't relate to BC. It's not Boston's team. If Umass-Boston were the main campus instead of a branch, I bet then you'd have more interest in college sports.