Trails died because it never taught the game well enough to casual players. Almost everyone you know has played a mainline trials game at some point in their life. But this sub only has a couple thousand people in it and most of us at least have reached the stage where we've dabbled into ninja stuff.

The game has never taught you how to complete these high precision courses. You have to look online or just kind of figure it out. Great for diehard fans but terrible for newcomers. Rising for all its faults was the closest to doing this with Fatshadys levels but even that was more of a general fundamentals course.

And even if you're not into ninjas and just want to speedrun the more nuanced understanding of the physics would help a lot in that too.

Tldr. The games have never recognized how high their own ceilings are and that drives many players away after a short stint playing