The difference lies in the ground contact time. The longer the foot is in contact with the ground the more time the for the muscles to develop maximal force. This relationship can be seen on the Force/Time curve. Because the gct in the start and accel is longer there is more time develop these forces, thus making maximal strength of primary importance especially within the first 10 meters. As the speed progresses the time to develop this force is less, therefore an increase in strength will have less transfer to distances near or around Max V.
With that said, strength is one of the easist fitness qualities to develop versus elastic or reactive power which is seen in Max V, which is why it tends to be easier to improve acceleration.
But, although it is easier to improve the earlier stages of acceleration, the improvement itself is relatively small as the distance is short. As Charlie would say, why waste so much time on the start to try and cut hundredths off your time, when you can take tenths or a whole second off concentrating on the latter portion of your race. (or something along those lines, but you get the point)