because neck cranks/spine locks like heel hooks are an extremely dangerous submission. let's put it this way: Do guys get their elbows hurt from armbars? Shoulders from shoulder locks? Ankles from foot locks? Ofcourse no matter how carfull you are sometimes when you "go at it" people get hurt. The difference between a neck and an elbow is that you neck/spine is so tricky that just a small tick could put you away from anywhere from a few days to years. That's why neck cranks (especially for beginners) are frowned upon.
I really love it when people say "oh i teach neck cranks to beginners and no one get's hurt" Oh really? What you mean to say is that "no one got hurt YET". People have sore elbows and shoulders/knees/ankles from submissions. Elbows pop and you don't think that your neck won't get injured?
Quoted for truth
People have families and jobs. Spinal injuries tend to get in the way of that - and last a lifetime. Its the same reason we don't allow knives and baseball bats into sparring ... they're effective, but that doesn't mean you want to spar with them.