I think price is the driving factor there. Toronto-area residents fly to Florida from BUF to obtain usually-cheaper U.S. domestics airfares while flying from YYZ means international taxes and Customs and Immigration user fees at both ends.
Side note: I often visit Buffalo over the U.S. Thanksgiving weekend but have been flying in and out of YYZ the last few years (not last year). With it being a U.S. holiday weekend, BUF fares are outrageous while it's not a holiday in Canada and YYZ fares are much lower.
But anyway, yes, I think Canadians would fly Delta YYZ-MCO if the service was offered.