I would imagine that the bias has to do with climate (which was a big problem for the NPPL last year), expense (mostly of venues, but also of player amenities like hotels and restaurants), and proximity to airports.
It seems to me like the Eastern US is rainier and more expensive than the West. More population density makes for more expensive venues and more legal restrictions (the health codes in Wyoming are not what they are in Miami).
All the same, I feel like the series should be as evenly distributed as possible. Everyone should have an event close enough to them that they could drive. Ideally, there would be an event in the Northeast, the Southest, the Midwest, the Northwest, and the Southwest. I'd like to see Philadephia or NY or Pittsburg, Orlando or Miami or Tampa, Chicago or Denver or St Louis, Seattle or Portland, and Huntington (or LA or San Diego).
The NW is actually really nice in the hottest part of the summer, it would beat the hell out of Chicago for a July event. Chicago in the early summer or early fall would be nice, before or after the main humid heat comes through. Huntington is almost cold at the beginning of the year, it might be better for mid summer, when other places are really hot. It seems like it would be possible to follow good weather around the country and spread out more evenly.
I guess that there is a fair amount of lobbying that goes on to get tournaments in certain parts of the country, and a lot has to do with what venues can be had.
Baca-Denver is about 40 hours drive from the left coast, about 45 hours from the right coast. It is somewhat more west than east, but it is about as close to Southern California as Chicago is to Tampa. I see what you are saying about everything being West, and it is West of center, but those two states and those pesky mountains make at least a 2 day drive.