Minor league hitters, on average, don’t have the same level of plate discipline as MLB hitters, aren’t as good at fouling off pitches to extend an at bat, and, well, get fewer hits. There’s a reason many of them never advance to the next level. All of this can result in a pitcher throwing fewer pitches per inning.
It’s not like there is a hard limit attached to number of pitches in an IP. And it’s the pitches thrown that matter…