Notably, Digg updated which also involved a worse interface and didn’t have an “old Reddit” interface you could access. Going to a site that was like the old interface involved leaving Digg and joining Reddit.

That is likely why you can now access older Reddit interfaces. They feel that many people will stay if they can find a way to use the new interface (and they may be right about that). The Digg approach of forcing all to use the new interface was a step over the line for Digg and Reddit likely fears a similar thing could happen to them.