I did some work for a client who wanted to build an app for it. After that I ended up building a few apps just for fun, and once they started bringing in some ad money, I just ended up building more.

I think their ecosystem is still small enough that good apps do well, whereas with iOS/Android you really have to build super polished apps and market them to thrive.

KaiOS is just HTML/JavaScript, I ended up using Vue and building my own component library that I could re-use in every app.