I suspect it's not out of choice but because parts of the new UI is duct taped to the old. That said they'll still likely rip it out soon and break everything in the process, just as Spez's idol did to Twitter.
I've had people working at Reddit tell me that old.reddit is so buried in there its removal would be hard. Which doesn't change the fact that they would pull it out of necessary. I'm guessing they'll find the routes for it and 301 anything to the new reddit. Easy enough change.
they probably already have all the mapping for the old system stored somewhere the first time they have to switch to old reddit. Just need to change the behavior of that to return a 302