It was originally set by automobiles:
Television has always been an advertisement-based platform and when television began, the only major national advertisable industry was cars. Since every year new cars were being introduced in September, September was when the television season would start. There was no reason to put up new shows in the summer if all the major ad dollars were coming in the fall. And it just stuck…
But that's changing:
Where before advertisers had the biggest say on when we watch television shows, now television has to work to shape itself around its audiences viewing habits. Just like the networks having to figure out a way to make money off DVR and Hulu views, they will also have to adapt to audience looking for summer content. It takes television networks, after decades of stagnation that correlated with decades of innovation elsewhere, accepting they're not in the automobile industry anymore.