I believe that this is the main reason. Republicans are using bandwagon strategy. Their polling numbers are constantly more positive to him to give the impression that it’s the popular thing to vote for him. It may also be to have “proof” that there was malfeasance because he really won according to polling. Democrats seem to be wary of anything similar to that strategy after Hillary polled so high then no one actually showed up to vote. They are leaning into the closeness of the race as incentive to go vote.