You are talking about a movie made in 1972, that no one yet had confidence would become the classic that it is now. While the 'Jane' scenes were filmed, any reference to child sexual abuse may have affected the movie rating and therefore the theater distribution.

All in all, it was wise to leave it out because its omission did not really affect the overall story. Vito Corleone got his way and Johnny Fontaine got the part, in a thorougly believable way without having to include the sexual favors of a young girl, except for referrences in Woltz's own story (which pretty much admitted revenge as his reasons for wanting to ruin Johnny's career).

If we're thinking in today's mindset of anything goes PLUS extended profits from pay-per-view and dvd sales, then of course that stuff would've been directly referred to without hesitation.