Garlean Inability to use magic.

[Gemina]

Google fails me in my search for why Garleans cannot use magic.

I had assumed it was genetic since it is passed on to their children, but it cannot be the physical body that is the limitation. Otherwise Elidibus would not have been able to use magic in Zenos' body.

So that makes it a matter of their soul. Which also opens up the concept that traits of a soul, such as the inability to use magic, can be passed on to children.

Is there anything in lore that goes over this more in depth?

There is also the possibility that Elidibus changed the body somehow via magic to undo the limitation before going into it. That could open up some interesting plotlines if someone is able to find the switch inside the Garleans to turn magic access back on.

Does anyone here think this will be explored more in coming game content/expansions or just a case of "They can't except when we want them to. Don't ask questions"?

I wouldn't use Elidibus, or any Ascian. They cheat, and have been doing it for a very long time. It is even explicitly stated that Ascians/Ancients being so aetherially dense, cannot weld dynamis. This revelation came in EW. It's a major WTF moment because Elidibus is clearly using limit breaks when we face him as the WoL in the Seat of Sacrifice.

As you pointed out, it is canon that the Garleans cannot use magic due to their genetics. What remains a mystery is why that is the case for them, and even this basically comes down to the Garleans being given the short end of the stick in nearly every aspect of life you can think of, which is also canon. What we learn about the Omnicrons is very parallel to what the Garleans have had to do to survive. Unfortunately for the Garleans though, we have plot armor. Otherwise, they would very likely conquer all of Etheirys. They were well on their way until we showed up.