I see...
So then why do the majority of trombone players sound that much better on baritone?

Is it because the bore of the baritone hides mistakes that have to be corrected on trombone (they are obvious) in embechure?

Because the trombone's sound is much more crisp and almost raw, to get a good sound you have to be that much more careful. So basically yes, a baritone/euphonium may sound "prettier," but to get that pretty sound on a trombone will allow you to sound amazing on a baritone/euphonium.