Pretty simple, elections are about numbers, not enthusiasm. 20,000 voters at enthusiasm level 100 does not beat 200,000 voters at enthusiasm level 1, even though the former has a 10x average in enthusiasm. Heck, HRC will have at least one dead voter for every DT rally attendee.
What I find odd is DT having a rally in Tampa. That is at least three now. What good does it do to speak to the same people who are already in your camp multiple times? Ego? Press? Why not spend some time where you might drum up NEW votes? At least this time his rally wasn't, literally, held accross the street from the failed Trump Tower "project", like at least two others were.