Because there are no democrats in the Democratic field.
Donald is kicking butt in a crowded republican contest. Imagine how well he’d do if he had NO serious party challengers.
What's to say if he were running as a Democrat, and was leading heavy in the polls, that many more would not join in the race as the Democrat contender? Just because he is leading the Republican side does not mean that would necessarily carry over had he announced as a Democrat contender. Though I suspect he would be enjoying the lead over there as well, as it is his message, not him that is resonating with the people who are supporting his bid.
We live in interesting times for sure.