A while back(sometime this year), I submitted a request for a speedrun leaderboard for the demo of a game. I checked today, and saw that one of the most actively run games was a demo. What's up with that?

What games are you referring to

Probably "Ghostrunner Demo", which is marked as pre-release. What is the problem with that?

demos normally go under a couple questions when being decided if they should get leaderboards, such as "will this game get a full release?" "is there enough gameplay in the demo?" "should the leaderboard be put aside until the full game releases?" and a couple other questions.

also, some demos were accepted a long time ago that would probably not be accepted now, due to changes in the request criteria.

That is why some demos are on the site and some are not.