Do the games in it have to be popular enough or smth?

I think site staff decide if a series should have series mod. It's because some series mod will abuse their power to add some leaderboards which is unrelated to the series or some leaderboards that break the site rule.

Fangames, custom content series normally get mods.

Any series that have game added a lot of the time have series mods.

Every time someone asks about series, scratch always gets brought up lmfao.