My guess, would be that it's only an option for barbarians, who can't be paladins. While I think a barbarian paladin would be cool; that isn't very logical. They are fairly opposite, really.
As far as other races worshiping The Tribunal, that's probably frowned upon for being uncivilized, not in tune with nature, or not evil enough....
Also, the Paladin would get his powers from the cleric line/deity...and The Tribunal doesn't really fit that.
This is just my opinion though. I have no sources or facts to back it up.
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