Yes I don't think that airlines will be scheduling flights from any non international US airport to SXM.
Airlines are always thinking of the bottom line.
Demand from small airports may not make direct flights feasible.

Most people who have to connect to meet a flight for SXM would also prefer to do so at MIA or JFK and not via BUF.
Connecting via a smaller airport will also cost more as there will be less flights to do so.