>Selection bias. People who go to tech conferences make up a tiny share of all the people working in the industry.

Yes, but they're also more influential in the respective communities, and more probable as presenters to be writing stuff others use (e.g. this Firefox dev tool).

So they'll use what they personally use to develop.

>Also FWIW as a long time JS dev, it seemed to me like Rails actually was one of the major contributors to the macbook monoculture because Rails (in the early days) was popular with startups who also often focused on iOS apps first, which means they had no other choice than to use macOS for development.

I don't think that's the case - Rails confs/devs was mostly Macs before iOS apps where a thing (which happened 4-5 years after RoR was released).