My only guess is because they want to force AI scrapers to pay for access. And most scrapers probably work best on old.reddit because they keep changing new Reddit’s layout. As a programmer, if you’re going to code something to access a site, (and you don’t want to constantly have to rework it every time they make a small UI update) you’re going to use the version that never changes. So by forcing them to log in, it makes tracking the individual users (and thus, identifying potential unauthorized scraper accounts) easier.

Easier to track users and produce verifiable "x potential viewers " to present to advertisers.

They've wanted to kill old.reddit for a while. It doesn't support advertisers or tracking the way they want. Also RES is still a thing on it. They're too chickenshit to do so though, since the actual content generators and knowledgebase still use it. This is another attempt to slip the noose tighter

It's up to you on whether you believe it or not. But the reasons they're giving are that they've got protections going on when you access new Reddit that they aren't able to implement on Old Reddit. Again, this is coming from the announcement from the Reddit admins. It sounds plausible to me and I really don't care to argue this. I'm just passing along what they actually said.