Why isn't "QUO" an accepted Scrabble word?

[Queen_Tonya]

@kellner

I thought the rule of thumb was that foreign words are forbidden unless they are somewhat commonly used like native words. Usually quo is only found in complete Latin quotes like those above. I mean, it’s not a word quo you usually form an English sentence.

That was my understanding as well, until I started seeing words like aa, qat and aalii being accepted, while zen and quo and injustice aren’t.