Democrats don’t usually force shutdowns because the people most likely to suffer under a shutdown are federal, state, and local government employees, people who work in academia, and people in major metropolitan areas, all of whom make up a significant portion of the Democratic base. It doesn’t make sense to force a shutdown if your own voters are just going to blame you for the disruption it causes to their lives anyway.
Now there’s far more to lose, and Trump is going to get the blame for it. Shutdown for a few weeks to protect a few hundred thousand federal workers for at least a year is a pretty safe gamble when you’re not likely to get blamed for the short-term disruption anyway.
The short answer is basically Trump supporters are overrepresented in the watchers. Biden also got numbers in the high 60s.