[jessewmc]

I can think of two reasons:

A sort of Dutch Disease caused by Rails is probably the biggest one. When one killer feature drives the adoption of a language, everyone who is interested and good at the language is involved with that ecosystem, and it becomes self sustaining.

The other one (already alluded to elsewhere here) is the culture of monkey patching and meta-code. I worked professionally in Rails for years, and I really enjoy Ruby, but every time I had a problem or question that required digging into the Rails source I was tearing my hear out. Almost everything is a cascade of hundreds of single line methods built on an avalanche of DSL abstraction and depending on implicit monkey patching and other crazy stuff that's a nightmare to understand.

It's not a culture of readable code in sizeable projects, which is what you need if you want to be widely adopted by people who don't primarily write code for a living (scientific work, other general work). Ironic considering how expressive and beautiful Ruby can be.