As a role-playing game author, I am mostly driven by my need for shorter texts. I play something I like and I wonder whether I could make it shorter, keeping just the ideas I like, maybe mix in some ideas I've seen elsewhere, and call it a day. As such, my games aren't very original but they are very personal. These are the games I like to run.
Another big driver is the need for games in German. Many of my games therefore started as "short German player-facing house-rules" – the rules I thought were most important, in German, and short.
Then the games get translated into English and sooner or later the games start to diverge because I use them with different groups, at different times. My hope and my curse is that the games are driven by the needs of my tables.