Attempts to advance the ball leading with the crown of their helmet the probability of a collision to the head goes up dramatically.
In a game of field position - literally the definition of a touchdown - why should offensive players be allowed to dive leading with the crown and defenders not?
When a defender hits someone in the head and the offensive player in a not leading with the head the call is easy. When an offensive player is trying to advance the ball leading with the crown of the helmet head-on collisions are inevitable.
Why call a QB down where they start their slide? Otherwise, player safety and tactical advantage are in conflict.