It's the extra layer of polish and presentation vs checklisting shippables but not doing much else
PSABR was a very dry presentation, with not so great onboarding for the general public. It's interface and gameplay was made to cater exclusively to small segment of the Smash community with a "let's make sure people don't think we're outright cloning Smash for the sake of it via some weird mechanics", and the sales reflected it.
PS Classic is the same thing. It's just an emulator in a box like the NES/SNES classic, but had issues like using PAL50 versions because those had translations ready to go so that Sony could skim money by shipping one device worldwide. Even Nintendo listened to re-shipping PAL50 way back and finally stopped doing it, you'd think a tech-aware company like Sony wouldn't do that. Combined with it's dry presentation much like PASBR, it's not very 'fun' to interact with outside the games. shipping it with the original PS controller instead of the dualshock in what felt like a blatant reason for them to do another one with the DS to get you to rebuy another one was the icing on the cake.