TL;DW: Zack posits that the additional mass of the hot stage ring pushes the total mass of the booster close to (or over) the limit of what the booster can land with, based on the amount of propellant which can be stored in the current design of the landing tanks.
The video is much more detailed, and definitely worth a watch!
Update: it worked, and the views were great
I’m sure that this is just a temporary thing until the ring is fully integrated into future booster versions. There’s been changes to Booster that’s bumped its mass up quite a bit I believe.
I’m sure one of the videos from the known commenters noted that this means it’s a fair bit heavier but future ones reduce weight. Due to how fast they iterate and build the changes are being applied to a few boosters from now but they’ll just make do with what’s coming before that.