Reddit wants lots of growth (they are taking new money now I think) and that means lots of new users that don't know the old UI. The new one is designed with those users in mind, not ones that know the site inside and out already.
Integrating RES into the main site would've been most of what they needed to get going...
in the current political climate, it's not hard to see why reddit might be cautious about implementing such features. of course, the real reason might just be that most reddit users don't even have one account to manage.
They need hundreds of millions of new users and those have to come from Facebook and probably on mobile.
The site isn't mobile-friendly though, is it?
It's slow, bloated, and has repeated interstitials demanding you get their app if you deign to use mobile.
How is this an improvement?