The same is true of Phoenix in January and Florida in April. Both were started by Barrett Jackson and now all of the others are there at the same time. It does make it easy to hop from one place to another if you are looking for a particular type of vehicle.
The article by Colin Comer is spot on. There are simply too many auctions going on at the same time in Monterey. Back in the early 2000s, the auction houses helped collector car prices but now that they have flooded the marketplace, it will have a negative impact on prices moving forward.