Probably it was created by someone who prefer mobile environments and who also believe that bulky big interfaces are good. New design seems to be aiming at keeping users engaged with the site all the time by obstructing visibility of comments, for example
No point designing it for mobile browsers if they're going to block mobile browsers from using it, or at best irritate the hell out of mobile web users with relentless 'app nags'
Yesterday I saw a thread on tildes where some new user presented changes to the site via custom style which introduces rounded corners and big elements all around: https://tildes.net/~tildes/16cl/i_made_a_thing_to_make_tilde...
Guess it's the generation gap - those who were growing up with mobile devices are fond of similar interfaces and that's what reddit tried to exploit
I'm on the opposite side: I prefer minimal interfaces, like old reddit, hn or current tildes. Lemmy seems to be more "modern" on that aspect