Because it’s not an alternative to people with YouTube as a job. It’s great if you want to have a couple of videos hosted and watchable by others. There is no way to monetize them by their very definition and mission statement. Their own website says it was “created for non-commercial purposes”. It’s his job though, so he’s not the target demographic.
You could integrate sponsors, which he doesn’t do on YouTube either. Or redirect to patron or similar services.
There’s also no (or very little) discoverable for people who watch sometime similar, which YouTube actually does extremely well. So how do you grow your audience?
Again, he doesn’t do sponsorships and doesn’t want to. Missing out on “only” AdSense is one of two revenue streams (the other being donations/patron).
And you’re conveniently ignoring the other points about discoverability. There isn’t any on peertube. So you can’t even grow the last remaining revenue stream.