I always assumed that the new reddit UI was more for app based usage than browser. And there is/was a hope to steer more people towards using the app.
I also assume they are smart enough to realize that some people will simply never use the app. In which case they're not realizing as much benefit from forcing the new UI on them. Sure, there's a cost to maintain the old one. But as long as things aren't breaking it's easier to keep it.
