It's fairly arbitrary. There were changes between Rails 3.0.x and 3.1.x that broke many popular gems (e.g. Devise). A lot of people get stuck at that transition, because it's difficult and costly.
This just seems like laziness. At the very least, you don't just suddenly choose to end-of-life a version of software that's still in wide distribution. Give people a bit of notice!
You really think that's a fair response? There was a parenthetical mention that "this will be the last release of the 3.0.x series" in the middle of a blog post about a critical security fix on the 28th of January. I don't recall any notice before that.
Even if the announcement weren't buried, most of us have development schedules that don't incude the spare time to completely upgrade our infrastructure with two weeks of lead time.
There's no real reason that this couldn't have been announced a few months in advance. I know that this is an open-source project, but one of the costs of having the immense privilege of users who depend on your project is that you make real efforts to give them a little notice before you deprecate their world.
So there's a good reason to support both x and x-1, but less of a good case to support y, y-1, AND y-2: those people can more easily upgrade to y-1.
So, as you can see, it's not arbitrary: x and x-1 are supported, y and y-1 are supported. Just no series as of two revisions ago.