Are you talking about https://www.speedrun.com/doom1 , "Ultimate Doom" from 1993? Looking at the leaderboard as a whole, and mostly on the full game categories, this is an active game with lot of runners submitting runs over the years, and several runs were submitted in the last few months as well, so surely this is the main site for the game.
If you look at IL leaderboards, some of them do have lot of runners, but some have only one runner as you said. This is because one cause only - only that one runner was interested in running the level, whatever the reasons for that are.
Running ILs and running the full game require different mindsets.
Doom is one of those special cases where they do keep tabs on records through their own webpage if I recall. I'm not into the series so I can't remember where that would be, if you join their discord im sure you can get some answers. @AltimaMantoid
