The first reason there is not much point, you could easily use atom (or even emacs) if you absolutely need a gui (both with built in syntax highlighting for ruby). However most rubyist are happy with the command line interface and you can run sketches directly from vim.

Perhaps the major reason is that JRuby provides an absolutely solid way of working with both java and ruby codebases (so there is absolutely no place for ANTLR or any other preprocessor).

Ruby has the brilliant rubygems package manager for distributing code.

PS: I would be very much like to be proved wrong but I very much doubt it. Despite Fukuda-sans previous work.