So, I've seen some runners doing full game runs on Twitch, Youtube etc. but the leaderboard is pretty much empty. Why? The game is really good for speedrunning imho, but people seem to be only concerned about level runs. The only reason I could think about is that after you finish a level, moving to the next level takes some time, but still it is not much of a thing. So, can someone explain why full game runs are not popular?

Because there's a million sub categories and people are torn on what to run. Some people like any%, others like SA or SA/SO. There's also not that many people running and the majority of the youtube runners don't really interract with us on SRDC, Discord or even Twitch to some extent.

Just to add, sometimes one of the levels in a full game run is either glitched (such as the level Patient Zero; SA and SA/SO are broken because of a guy jumping off a mountain at the start), or just way too hard for the category (again Patient Zero, SA/SO on that one is almost Kaizo levels of masochism to pull off, but the other levels in the campaign are fine).