Displacement has a longer animation lock. Delaying your GCD is the same as losing potency, and Red Mage has beefy gcds. Triple weaving to use the corps to get back into melee range to start your combo is losing significantly more than the 50 potency on engagement over displacement.

The rest is likely "math" and I don't have those numbers for you. Likely to do with raid alignment and making sure your heavy hitters are getting the buff / lining up, which is a significant gain over the course of the fight and likely doesn't lose any uses.