Because it is the saddest time of the morning for him. He thinks... another day of looking after those silly chooks.
Seriously... A rooster's crow is pretty much the same as a bird's song, and is exercised as an territorial warning to other roosters as "this is my patch if you don't heed this warning there'll be a scuffle". It is mainly done in the morning because the air is usually still so the 'song' carries further, and it is a way of re-establishing himself as the Master of his harem. The crowing is not strictly limited to the mornings though, after a fight or chasing a threat to his flock they often crow.
The hens don't have the right physical structure in their lower larynx to make the sound, pretty much as human voices are usually lower with males. In a flock where there is no rooster present an alpha hen will take over his role as dominant protector and will even attempt to crow, wich as an most unusual sound. Those dominant hens will even mount other members of their flock as a rooster would, this is referred to as pseudo-copulation.