Rails took off in 2004 as a rebellion against the overly-heavy Java frameworks like Struts. But now what good ideas Rails offered have been incorporated into languages that run on the JVM, and a lot of innovation, in particular with concurrency, is happening on the JVM, and some of those things are areas where Ruby (other than JRuby) is weak.
Even the cutting edge of Ruby is now on the JVM, as Rubyists see a need for threads and find jRuby the most convenient way to move forward: http://tonyarcieri.com/2012-the-year-rubyists-learned-to-sto...
Rails took off in 2004 as a rebellion against the overly-heavy Java frameworks like Struts. But now what good ideas Rails offered have been incorporated into languages that run on the JVM, and a lot of innovation, in particular with concurrency, is happening on the JVM, and some of those things are areas where Ruby (other than JRuby) is weak.