The high-minded reason is that they think that Parliament sitting in September will help show that this new Parliament is different from its predecessor, more diligent and more focused on getting things done. The low one is that the Tories want to get as much done as possible before a new Labour leader is in place, the thinking is that Labour will not have a new leader until after Labour’s autumn conference.
Berocw’s relationship with the Tories remains uneasy. I suspect his fate turns on the size of the Tory majority. If it is small, Bercow will probably be gone. Cameron will need to throw some bones to the right and letting Edward Leigh challenge Bercow, which he is said to be keen to do, would be an easy thing to offer. If the Tory majority is big, I suspect Cameron will try and persuade the party to leave the Speaker in place, arguing that there are more important things to do.