Which things in particular? Rails supported both Ruby 1.8 and 1.9 simultaneously from Rails 2.3 through Rails 3.2, without it apparently being a huge struggle for the developers, dropping 1.8 only at about the same time that support for it was being dropped by the core Ruby devs altogether. And other posts from maintainers of smaller, but widely used, gems describe it as being a bit of a bother to set up, but not nearly the struggle that it took to run anything in both Python 2 and Python 3. See, e.g.,

https://thoughtbot.com/blog/the-journey-to-ruby-1-9

One key here is that the common subset of Ruby 1.8 and 1.9 was large enough that you could ship a single library for both, and a lot of large ones did, as in the above examples. That wasn't the case initially in the Python transition, and by the time Python3 had started making enough concessions to back compatibility to make this practically possible, people were a lot less willing to try.

(I personally was managing upgrades or a large Rails app at the time; Rails upgrades were, and remain, a bear, but as an accident of code style which just avoided the tricky spots anyway, the changes required to move that code between any two Ruby versions have never been more than a day or two.)