It’s to make censoring the front page easier, as /popular is sorted by sub popularity, not post popularity. /all was sorted by post popularity, weighted by how many users were in a sub vs how many interacted with the post. For instance, on /all, a post with 1k/1k active users upvoting it (a 100% upvote rate in the sub, but posted in a small niche sub) would get sorted above a post with 5k/25k (a sub with literally 25x as many active users and 5x as many upvotes, but only a 20% upvote rate).

/all has pretty consistently been a thorn in the admins’ sides, because wide scale censorship is really difficult when a tiny niche sub can hit the front page of /all. In fact, /all was fantastic for new subs trying to get off the ground, for this exact reason. If you were trying to start a new subreddit, one of your biggest sources of new users was likely going to be /all users. But it also means that wide scale censorship was difficult for the admins, because any splinter movement could start a small sub and start hitting the front page. Admins delete the sub? Users just make a new sub and repeat.

Removing /all (and forcing users to /popular instead) allows the admins to direct their focus to only the big subs. Because /popular is sorted by subreddit popularity, not weighted popularity. So small subs have basically no chance of appearing there. By pushing users to /popular, they can focus their attention on the large subs (which largely have mods who are cozied up with the admins already) instead of having to moderate a thousand tiny ones.

It’s no mistake that they removed /all right before the midterms, as they can more easily push a certain narrative if they only have to focus on a few subs. It’s also an advertiser thing, as /all doesn’t filter out NSFW or controversial subs by default, but /popular does. Advertisers tend to complain when their ads are shown alongside NSFW or controversial posts, so directing users to /popular allows them to tell advertisers that they’re not accidentally showing their ads next to porn or political posts.

I think the curated front page is a death knell. It certainly was for Digg.