It's not about affordability or car ownership. Its about driving in Boston sucking in general, really sucking in the vicinity of both casino sites, and people from the area knowing that. I still haven't heard an answer as to why people/tourists who take the T to do other things in town (symphony, concerts, museums, clubs, fancy dinners, sports, whatever) are going to not want to do that to go to a casino.
I also haven't seen an example of a casino in an urban area with a large transit riding population for comparison. Vegas, Atlantic City and Foxwoods are terrible examples. People drive there because they have no choice, and even then there are casino buses. I don't gamble (because I'm really good at wasting my money all on my own), but my friends do. Every single person I know who drives to Foxwoods complains about having to drive there, and says that if they didn't have to they would go more often. My girlfriend and I both own cars, but I take the T to the Cambridgeside galleria, and when the casino opens I will be taking it to get there to check it out too.
Marketing to the local populous, however, allows him to tap into the existing pool of college kids, conventioneers, tourists, and residents. It doesn't fill hotel rooms or leave people sitting at a table or slot machine for days at a time, but it does guarantee a steady stream of money coming in from people looking for something to do on a saturday. It also requires a very small investment to tap into: some sort of connection to the existing transit system. These are people who are already here and are looking for something new to do. He just has to get them through the door.