Although I enjoyed the question, and wrote an answer, I can't for the life of me see why the question about magic-using kings isn't a clear-cut case of idea-generation.
"I can't think of a single, dominant reason...."
"What would be so widespread...."
It seems clear that the question boils down to this:
Given the following setup
Please provide ideas to justify it
I see no way to evaluate results other than the OP's preferences. The conditions specified are not "too broad" in the strong sense, but they're wide enough that a great many equally valid answers are possible.
Am I missing something?