User blog comment:Odisseas25/Why does the AP go to mid?/@comment-68.144.165.245-20130210043050/@comment-75.70.71.85-20130210051113

This is pretty much it. Going mid gives access for the AP carry to go to both lanes easily, and often both bot and top are going to be some form of AD. Also, they are often very squishy so the shorter lane makes it easier to get out of trouble.