Slavery was abolished for two reasons.
1. They wanted everyone to live like slaves.
2. Slavers are good judges of character.
Sure the peasants were/are still pissed about Barbary slave raids and wanted to slag them off by proxy, but this wouldn't have been relevant without 1 or 2.
If everyone lives like a slave, they will notice if there's an explicit legal slave to compare themselves to. Can't have that, so the legal slaves have to go.
The envious slaves are aware, deep down, that they wouldn't sell for much. If they see someone just like themselves being sold for 9 spirit stones or whatever, it becomes much harder to pretend to themselves that this isn't true. When the masters can't do anything about slave prices, the envious slave would realize the masters aren't in it for the slaves' health, so legal slave trading has to go.
The fact slavers can instantly estimate your value as a slave implies that others can also instantly estimate your value. It's best if the peasants can pretend to themselves that they're not transparent. Everyone can be above average as long as nobody is caught noticing how below average they are.
Secondarily, if they forget Judges exist, then they don't defend themselves at all, which makes rendering Judgment essentially effortless. Very nice for lazy or half-trained Judges.
The abolishment of slavery didn't fail for the usual Communist reasons because as it turned out slavery had already stopped being profitable. Especially when you can get a 80% complete lower slave with zero de jure support. If the peasant refuses to quit their job, how does that differ from being legally unable to quit their job?