I think the general idea is that people have gotten older and there has been a lack of upcoming content creators to let's say "replace" the ones that got older and don't have the time to make stuff like they used to.
Like @erhminer said, I think there's a lack of tutorials for people to get started with making stuff. I like @monster jam fan's track making tutorial but I haven't actually used it yet due to lack of motivation for making stuff.
And this is just speculation, but maybe one of the things that's holding people back from making things (like mjfan posted above and myself) is that they wouldn't be utilized. There's lots of tracks on the site that were made just to be used for a certain league. If you spend a week's worth time (idk how long it takes to make tracks on average so we'll just say a week) making a track for a league and the league doesn't even happen (I've seen this happen before but I won't name names) or the guy who's running the league doesn't even like the track, you feel like you've wasted a lot of valuable time that you could have used on something else. Same thing if you make your own custom truck for a league and the league doesn't start.
I think the community here would benefit from a "comeback" league or something. Nothing over the top, but just something to get people back into the game and to get new people interested, and maybe to get people like me who have been here for a bit but haven't really done anything. I would prefer someone who has been here for a while and knows what they're doing to run it, but that's just an idea.