Because sprint stages don't effect GC and so, in the economy of a three week race, the teams with potentially winning sprinters must be "greedy." And such stages are less of an audience draw, so it doesn't take much out of the till when one sprinter dominates, but actually adds to the introites. By contrast, if every GC stage goes to one rider beyond the necessary that team has its hands in the till too much and folks will start to lose interest in the final outcome, which is a foregone conclusion. In a sport where nothing happens of significance for most of the race, killing the suspence is not good business.