There's actually a quite simple answer to this: iPad's are too profitable. That may not seem like a problem, and it certainly isn't to Apple, but it's a problem

There's actually a quite simple answer to this: iPad's are too profitable. That may not seem like a problem, and it certainly isn't to Apple, but it's a problem for the market. When it's easy to make huge profit margins and massive sales volumes with relatively simple designs there's not much impetus to innovate, take risks, or pursue smaller markets. The potential opportunity cost that Apple faces in spending a lot of development effort as well as production volume on an "iPad Pro" would be enormous, and almost certainly more than what could be recouped in profits even if such a thing were successful.