Because weird situations in the rules that no one sees regularly are not often in your weekend amateur umpires knowledge base. The vast majority of people in the game (players, coaches, umpires, parents, etc) don't hang around in forums such as this or do research and study etc, etc...But they did not call the pitch illegal. I was there and saw it happen. Our batter was out as it was the third strike. And I saw her pitch it the day before. Why wasn't it called illegal then?
So in the end, the same reason your coach didn't protest and get the UIC over to make a ruling.