Why is the Loire Valley one region

My wife and I just had a wonderful visit to Loire Valley. We were based in Tours and made it to Chinon, Bourgueil and Cheverny. The Loire is so big that it seems like the individual AOCs have nothing in common with the AOCs which the further away. For example, what does Sancere have in common with Vouvray other than the river? They're about 100 MI apart and use completely different grapes. Is it the soil and the climate?