It's possibly due to something legal. Letters Patents are an alternative form of legislative power to the normative Acts of Parliament, so it wouldn't surprise me if there are established limits to what can be done with them.

It also wouldn't surprise me if there are no formal limits, but there are effective limits. Quoting from wikipedia, "Clearly today parliament tolerates only a very narrow exercise of the royal prerogative by issuance of letters patent, and such documents are issued with prior informal government approval, or indeed are now generated by government itself with the monarch's seal affixed as a mere formality."

So, its also possible that there are powers other than Queenie herself holding things back.