Probably for reliability and stability — otherwise, every view from every federated instance would create a new request to the hosting instance. The protocol itself would basically DDoS smaller instances. Also you can still read the cached version on your home instance if the remote instance is temporarily down.
It’s unfortunate that a handful of replies here are demonstrating exactly why the Beehaw community leaders felt they had to make this choice. 😞
If Lemmy instances are like web forums, federation basically gives us a “Sign in with your [home instance] account” option. (That’s not technically accurate at all, I’m only talking about the user experience.) It reduces user friction and helps people participate more widely. They just stopped allowing that from certain instances because they think adding a bit of friction back in will be healthier for the Beehaw communities. If you’re on one of the defederated instances, you aren’t banned. Yeah, it’s inconvenient for you, but you just need a different sign-in (at least for the time being).