Because the "new" layout is awful.

Yes, I agree, but the old one is unusable on mobile, and visiting old.reddit.com forces me to keep using it until I logout and login again, which is also awful

The new layout is awful on desktop. Change "old" to "www" and you'll be all set on mobile.

That doesn’t work, but that’s not your fault! I think it’s setting some cookie or something, which makes this annoying to deal with. Anyway, thanks for the reply

The option to change it is buried in the settings.