Ajani was just pretty bad overall, especially for 6 mana, but the B/W sorin got played for awhile. I don't know exactly when it stopped being playable, somewhere between kaladesh and Ixalan the meta moved away from the kind of deck he fit in, but he wasn't a bad card. Plus, on the surface he seems more 'tricky' then vraska to evaluate. His +1 doesn't have an immediate impact, and his -X can't kill everything. Making a 2/2 or immediately destroying a permanent (most of them, anyway) while ramping 1 mana seems more obvious.
I can understand it not having a place if the meta was super fast, but midrange value decks have been big for awhile now.