Which is real shame, because there are many really good options for Ruby Web development.
What people are saying here abut Rails I felt about Nitro, whihc came out at the same time but had a more well-considered implementation. It was thread-safe from the start, much faster than Rails, did less mucking around (if any) with core Ruby classes, made it easy to swap in different ORMs, offered a trnsformation pipeline that was simialr to Rack middleware; in fact, it seemed much like what Rails3 is going to be, only five years ago.
But it got no traction in Rubyland. In fact, at one MountainWest RubyConf, Chad Fowler flat out told people to stop working on Nitro, because there was Rails.
There's a weird myopia among many Rubyists, a lack of curiosity about what else is happening.
The Merb/Rails merge reveals some reasons why there aren't a lot of widely used Ruby frameworks, and I'd say it's mostly a large scale manifestation of DRY, combined with the maturity and wealth of features in the Rails codebase and the fact that most of the changes devs would like to see in Rails are already going into it.