Maybe they want to be on the winning side this time (just joking). My guess is that the trump campaign is spending on ads on YouTube, while the Biden campaign isn't. There seems to be a lot of mashups and videos comparing statements of Biden now and years ago. I get the impression the general tone of most of the videos on YouTube are negative for Biden, and probably not the best place to spend ad money. Especially considering that YouTube will start recommending political videos trying to get click rage engagement going on.

I made the mistake during the last election clicking on a promoted video about Hillary and my god I had to spend weeks clicking "don't recommend this channel" in my feed. YouTube will shove political video recommendation down your throat. I don't like political commentary regardless of what side of the ideological spectrum it comes from.