Last year the leading rookie receiver had 60 receptions. Leader in yardage was 793. Leader in TD's was 7.
I think it's becoming harder for them to really put up big numbers. I think there are some who contribute right away, but not put up the numbers generally required to win ROY. Especially since, and this was my original point, RB's can generally step right in and play since it's a much more instinctual position.