Mr. Katz advised in multiple interviews/tweets (if I recall correctly) to wait for 1.9.2, and the community tends to listen. Rails 3 was tested against 1.9.2, not 1.9.1; this provided yet another reason to wait.
To add a few more details, it wasn't until 1.9.1 was out that "normal" people really started to look at it. There were quite a few changes that took time to be properly absorbed by the community (e.g. Internationalisation, see James Edwards Gray's blog).
Most gems also only started then to consider 1.9.1 as one of their targets, but it took some time. And anyway Rails was moving even more slowly.
Personally I've been using 1.9.1 for about a year now, but I don't make a living out of Ruby (or Rails) so it's understandable that people who need more reliable support waited a bit.
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.
I moved Shoes over to 1.9.1, I agree, it's not bad. Don't underestimate laziness.