The MRI Ruby 1.8 garbage collector was sub-optimal for the large web applications it ended up powering, due to both blocking / performance issues and a lack of predictability and instrumentation. In 2009-2011, many high-profile Ruby 1.8 implementation enhancements focused on garbage collection, as the MRI 1.8 GC became a major pain point for a lot of growing high-profile Ruby + Rails shops. Both Ruby Enterprise Edition and Twitter's Kiji were very popular MRI 1.8 enhancements that focused on improving the performance and visibility / instrumentation / debuggability of the garbage collector.

Now that JRuby is much more mature and YARV / Ruby 1.9 has enjoyed wide adoption, more of the Ruby community are able to ignore GC (or only tune it once) - but that simply wasn't possible with MRI 1.8 and a large-scale web app.