That's not even true. Plenty of startups use the languages you mentioned. When Rails first got released, there was a certain wow factor - it made making sites very easy. There have been plenty of web frameworks released afterwards for other languages that made it easy to spin up a new web project, but Rails has remained ahead in terms of how much mention it gets, for various reasons: first-mover advantage, its commun…
I think the reason of rapid development is the reason of its popularity.
Except that you can develop just as fast in Python if you knew what you were doing.