[coldnebo]

Rubygems forms an ecosystem of libraries on top of Ruby. Most gems are dev/tested within MRI, some work across other Rubies, some do not. It's difficult to tell which is which. This means most teams choose MRI in much the same way that most Ruby teams choose some form of unix -- compatibility.

Ruby also suffers from the lack of a formal VM, so other implementations are merely similar rather than being guaranteed to run all programs. This is not the same with JVM vendors, where choice of IBM vs Oracle is largely based on extrinsic features rather than intrinsic compatibility. (Or at least you have to get in really deep to find differences-- in Rails you usually find incompatibilities that stop you from even starting the app).