Once a dev just got bored and rushed an ending
a second time a dev suddenly said his game was just a demo we paid for thankfully cheap.
a third time a dev released new versions for every update, I paid accidently for something I might have gotten for free as well, that I'm not sure about anymore, but here it still happened, that the dev just removed the version I bought.
a fourth time a dev hasn't released an update for a few years which is not something steam protects you from. 1 and 4 happened there.