The reason this site is dying.

[GeneralGuy]

A lot of people got their "make my own mario levels" itch scratched by Super Mario Maker and Super Mario Maker 2, both much easier than trying to make your own and learn some custom engine, so I'm not really surprised that activity has slipped.

You also have to consider that forums are a software no one likes to use anymore. Join the discord and you'll find it is much more active.

Also the fear of getting C&D'd or DMCA'd by Nintendo like with AM2R, Pokemon Uranium, etc. And the exponential rise of small indie game developers in the past decade would encourage prospective members to rather develop their own Mario-inspired indie game and make money off it.