The VST market isn't really lucrative. I have it on good authority that most plugins sell at most 200 copies. Music software is also very challenging to write because it combines multi-threading with soft-realtime requirements. So any programmer good enough to write non-trivial music software can easily make more money with fewer headaches writing almost any other kind of app. So I'd say the only good reason to write music software really is just for the enjoyment of doing it.

As for myself, as much as I enjoy writing music software I enjoy making music more and it seems like I only really have the mental energy for one or the other.