Selection bias. People who go to tech conferences make up a tiny share of all the people working in the industry. Plus they're far more likely to look towards other people going to those conferences for "trends" than to the majority who don't, thus reinforcing that bias.
Even most designers I know personally don't use macOS. But if I was going by what I see at JS conferences, you'd think Apple has a 99% marketshare (although things have improved in recent years with "full stack" bringing in Linux folks and Microsoft's investment in dev tools making Windows more appealing).
This is similar to how everyone at these conferences seems to have one of the latest iPhones yet (depending on where you live) the majority of smartphone users uses Androids and those who are using iPhones generally hold onto them for years before upgrading.
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. As iOS became less of a focus than web dev, the macbooks stayed and only now seem to be slowly replaced as Microsoft is making a heavy push into web dev relations and the opinion of Apple's quality gets tarnished by dev-unfriendly decisions and hardware problems.