I guess this is probably the reasons:
This is where Datashack and WSI are located. They're known to sell budget/cheap servers, therefore this location is generally pretty overcrowded in the VPS Market. Although personally I love WSI and Datashack especially since the servers I have with them are perfect for my use.
Mostly because CC saturated this area.
Similar to WSI and Datashack, this is where Dacentec is located. Now, Dacentec was previously single homed with Cogent but recently they added nLayer and in my opinion are decent for what it's worth. Same issue though with WSI and Datashack. Cheap servers at that location therefore pretty crowded.
BurstNET is here. Another budget dedicated server provider.
I'm assuming because so many people are in Seattle that this was put on the list? I can't think of any other reason (although I personally love this location).
Off the top of my head I believe this is BHS, where OVH has their new North America Datacenter located. Again cheap servers.
I think peterw just doesn't like locations that's frequently used by others. Although I don't see this as a reason to avoid these locations but merely just reasons to shop around. There's a reason why they're common locations (probably cheap prices and decent networks, I know RamNode has a great network in Seattle).