1. Time spent upgrading Rails is time not spent building awesome things for our customers to use. We're extremely customer focused.
2. Performance is also a huge concern of ours. Over the past few years we've driven response times down dramatically. Upgrading Rails not only presents a slower framework (Rails 3/4 is not faster than 2.3), but a different architecture — changing where our performance chokes are. We'd have to do a ton of work on this front just to keep performance steady. Again, time spent here is time not spent making our existing infrastructure faster.
3. We've spent the past three years working on upgrading the stack. It's not so black and white — we have many of the benefits of newer rails without needing to upgrade everything. There has been a lot of effort here.
In my own personal experience, upgrading frameworks is generally the fastest way to create bugs and work on the opposite of customer impact. I don't think you should feel like you have to be on the bleeding edge.