That's actually not correct regarding MHT and Boston's suburbs; there are several hundred thousand people along I-495 in the Merrimack Valley (Lowell, Lawrence, Haverhill, Andover, etc.) who are 30-40 minutes from MHT and 30-40 minutes from BOS (assuming traffic cooperates). BDL is the airport which sees limited traffic from the Boston suburbs, given that Windsor Locks, CT is approximately 100 miles from Boston by road. PVD is more convenient to Worcester than BDL, and Springfield can't be called a Boston "suburb."
BDL is a respectable market in and of itself, given that the population of CT is a bit over 3 million and air service to HVN and GON is quite limited. It's the most convenient airport for just about the entire state (excluding extreme eastern CT ,which is closer to PVD, and southwestern Fairfield County, which is closer to LGA), not to mention much of western Mass. and parts of southern VT.
As others have said, I don't see Song as being aimed against Southwest, but rather jetBlue. Delta wants to protect its market share and business in Boston, Hartford, and New York City, and this is how they intend to do it. They're not as big a player at PVD or MHT.