I can't speak for _everybody_, but I'm waiting to upgrade all of my sites that use Rails until Ruby 1.9.2 and Rails 3 both came out. I don't want to do two large updates, I'd rather do them together, and Rails 3 didn't support Ruby 1.9.1.

Also, there's just general conservatism; Ruby 1.9 was originally Ruby 2.0, but got renamed to 1.9 for reasons I don't quite remember/understand. 1.8 -> 1.9 is a much larger transition than the scheme would imply, roughly equivalent to the Python 2.6 -> 3 change, and we've seen how well that's gone. As time progresses, more people switch over, and each new release makes people decide it's finally time to re-write their scripts.