It's quite simple, the market for paid end user applications on Linux is tiny. Canonical was saying years ago that they were going to add paid applications to the Snap Store but seemingly never got around to it. Elementary's App Center has integrated payments but they are optional and ~99% of users do not pay anything.

You can certainly make a living selling a cross platform application that supports Linux, but it's probably quite difficult to make much of a living selling Linux-only applications.