For a lot of factions, a reigning queen (as opposed to a queen consort) would not have been possible.

The French started it in Christendom with the so-called Salic Law (which debarred women from inheriting a throne) and the idea was picked up in one form or another by many of the other states of the time.

I'm pretty sure that Islam debars women from ruling men but whether that is Quranic or just tradition, I'm not sure. There'll be someone on these forums who could tell you, I guess.

Finally, there were some states with reigning queens but the example you gave shows the problems inherent in that. Matilda's rule was opposed by an enormous number of her subjects - hence her civil wars with Stephen. After Matilda, the English didn't have another reigning queen for what, four centuries?

I think CA probably realised that reigning queens were a bad idea. Some factions simply couldn't have them and those that could might well be faced with constant civil war until her majesty popped her clogs.