My guess is that it's due to Mace being a 4* bronzium unlock character. That perhaps limits how good the character can be. Or the fact that jkr's kit stunted the touchups of other jedi due to balance issues.
Mace has also received 2 minor touchups in the past (damage increase to both basic and special, and another to make his expose irresistible). Not that either did much to improve his usefulness. Maybe if he had a 100% chance to inflict taunt and 3x expose on a random enemy for shatterpoint, leaving the character dazed or armor shred afterward if he crit, or some similarly better mechanic than a random expose he often can't target.
