More number of js frameworks there are for serving similar needs, more fragmentation there will be. Fragmentation leads to in stagnation in development process (slower bug fixes and feature delivery) and broken community support that can't gain critical mass.
Ruby community somehow avoided this fate, and almost everyone pushed for Ruby on Rails as the single dominant web framework of choice for Ruby. Sure there's Sinatra, but it's miniscule compared to Ruby.
> Ruby community somehow avoided this fate, and almost everyone pushed for Ruby on Rails as the single dominant web framework of choice for Ruby.
Well, that's not really true. There was considerable pushback against RoR as a web framework, leading to lots of alternatives, most notably Merb, and to Rack as an underlying platform for web frameworks.
Eventually, though, Merb and Rails merged, Rails incorporated Rack, and Rails incorporated a mechanism for running other Rack applications as endpoints alongside traditional Rails endpoints. Furthermore, while Rails is the dominant full-stack framework (largely, through having directly incorporated much of the competition into Rails), there's plenty of alternatives for many of Rails core components (e.g., lots of alternative ORMs to ActiveRecord.)
Ruby community somehow avoided this fate, and almost everyone pushed for Ruby on Rails as the single dominant web framework of choice for Ruby. Sure there's Sinatra, but it's miniscule compared to Ruby.