It comes down to the project plan for DLC, how much Sakurai has been budgeted, and the release schedule. At least, that's my guess.
As I've said to TPC and Arcanir my line of thinking is simply that traveling or negotiating that adds layers of complexity in the process...DLC is lower budget, on a timed release schedule, and there's smaller staff to boot. It really comes down to what expenses can be afforded, and if the end goal, or character, can provide the profit to make it worth it.
It really comes down to the questions of, "Which characters allow for the least cost, and the biggest profit potential?" All we can do is wait to see who gets picked for the remaining 4 (arguably 3) picks.