I think this is because, in the 2000s, when a lot of these companies were founded, Ruby and Python hit an pretty much empty sweetspot for web application development in between quickly thrown together scripts and the super heavy enterprisey frameworks.

But since then, that space has gotten a lot more crowded from all sides. There is now a pretty big selection of mature technology to pick from depending on your needs.

In the case of Python, it got also a massive boost from its popularity in datascience. Couple of years ago I thought Python web development is dying out slowly, but seems to have swung back quite a bit.