Redux

Simple...because proportional allocation of delegates prevents any one candidate, like a Palin or a Tea-Bagger, with the most and energized active base, winning all the delegates from states with only a plurality, but not a majority, of support from within their own party.

On a more general level, it also extends the primaries to give every state a voice...which, btw, was the reason the Democratic race between Hillary and Obama went to the very end.