The extended feedramps in carbines were necessary because of the shape of the M855 round compared to the M193 round. The M855 is more sharply tapered than the M193, if you compare the two bullets in the casing, the M193 is slightly fatter and more rounded.
While rifles seemed to be able to continue to feed M855 without problems, however, it was found that in the carbine, particularly on full auto, there were occaisional failures to feed, due to the different profile of the M855 round.
Colt discovered that by lengthening the feedramps into the receiver, they were able to satisfactorily eliminate the tendency of M855 to FTF. Note that Colt has used two different feedramp configurations, "carbine" feedramps in the 700 series, and the current "M4" feedramps.
~Augee
ETA: Primary source is TBR II.