Although I am just a audience, I wonder the reason of it. Is this the reason this DLC has much less runners than the main game had back then?

Yeah, I also been questioning this recently. By the way, BETON BATH DLC is far less popular than the original game, prob. because people don't want to pay money for additional crazy difficult map. And the other reason is that BETON BATH contains really questionable gimmick methods of speedruning. There is in-game leaderboard for BETON BRUTAL & BETON BATH, and yeah, there aren't even 1k players for the DLC map yet.

Anyway, I really want the leaderboards for BETON BATH to be added, cuz I already have some decent runs on it :)

Even as watcher, I want to know about this "questionable gimmick", it will be better with video.

Main reason is this page was made before the game had replays built into the leaderboard, and now with it built in there’s not really much reason to have it separated from the game