One big reason for moving to 1.9 is that 1.8 is hitting EOL in June[1]; this means no further security patches either.
(The plan for this was originally announced[2] in 2011.)
1. http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-dev/4...
2. http://www.ruby-lang.org/en/news/2011/10/06/plans-for-1-8-7/