Why? Or rather, Why Not?
Because that's the way the founding fathers decided it. Wasn't at the meetings so I can't say for sure. I would guess because it is a great books program and Aquinas is studied both for his theological contributions to western culture and his philosophical.
D'oh, thought about your question more and realized that I can name three other St. Thomas Aquinas Colleges, one in NY, one in TN and I can't remember the other one.
Probably wanted to avoid as much confusion as possible, so went with a slightly different name. But like I said, I wasn't there, never asked why.