Because it has to share gear with other jobs going forward and be used with all the existing gear. Holsters on the back waist are just going to be a design nightmare without putting it out real far and having a large gap, or clipping through half the armor in a horrendous way, and limiting design of future armor. Its the same reason all 2 handed weapons diagnal over the right shoulder instead of some jobs left and some jobs right. Same reason EVERY 1 handed weapon is on the left hip. Books. Mch Ammo. Swords. Or mirrored dagger/fists. There are only two slots they need to consider when designing gear. Diagnal over the back and left hip. Notice how Mnk and Ninja also share gear and are the only 2 jobs that have mirrored hip placement? Coincidence? New holster positions complicates the rather simple system they have built the game around for 5 years.
