I observed, as I've done before, that NASFiCs have many of the disadvantages of a Worldcon (one-shot, no history, no coherent marketing, relatively expensive) and few of the advantages of one (Worldcon is something many fans actively want to attend, while NASFiC is a "consolation prize" at best; many of the "usual suspect" middle and top management recruits go to Worldcon if they possibly can do so; even those who do go to both are terribly distracted and apt to give NASFiC lower priority).
A factor affecting western North America is Burning Man, which is Labor Day weekend, and which draws off a lot of fannish types. I know that ConJose had difficulty recruiting some very likely people who "always go to Burning Man."
In the Southeastern USA in particular but actually affecting the continent as a whole, and even with worldwide implications, is Dragon*Con. When D*C and Worldcon/NASFiC were on different weekends, there were a fair number of people who would go to both; however, when faced with the choice of one or the other, they pick D*C for a variety of reasons. I lot of professionals of various sorts, including artists and dealers, go to D*C even if it's more expensive and difficult for them, because that's where the money is. I know of one webcomic artist who lives here in the Seattle area and who at least of couple of us members of CascadiaCon asked to attend NASFiC. She's constantly saying how little money she has and how dire her personal situation is, and yet rather than come to the local-to-her con at Sea-Tac, she went off to D*C because that's where the people and money are.
I think it's time we Worldcon (and NASFiC) runners seriously consider abandoning the Labor Day weekend and move our target dates to weekends in August. We already have had several recent Worldcons, including this year's, on weekends in August, and it doesn't seem to have negatively affected attendance. The historical reasons we held Worldcon over the weekend ending with the first Monday of September are becoming increasingly irrelevant; maybe it's time to abandon some long-held opinions.