Comparing these two pages head-to-head...
en.wikipedia.org
en.wikipedia.org
...seems fair to say the GOP had a more open and competitive process, but not by all that much.
That's a good question, and there isn't an obvious answer.
If I had to speculate, I'd say that the Democrats faced a much steeper opportunity cost due to foregoing a primary process in a year where incumbency itself turned out to be systematically disadvantageous
across the globe. Dems had the chance to pick a relative outsider promising progressive reforms and instead chose
more of the same.