Yeah, I have to agree, November is a pretty terrible month for this event. Anyone involved in education, students and teachers alike, has a lot of extra work at the end of this month. And Americans all have the holiday pressures as well.
I suppose there are problems with every month, though. Many (most?) schools start at the end of August, so I don't think that would work well. December is obviously out. January might work, but doing anything in the middle of winter is asking for extra stress from depression.
I'd say the best month for this would be July, when I don't have any work pressures, but a lot of people travel during the summer, so that wouldn't work, either. Or maybe October. On the other hand, they do have Camp NaNoWriMo during some of these months. It's not exactly the same, since you set your own word count, but still, it does exist.
January: Too dark and cold to focus
February: Too short
March: Spring Break for many people, travel
April: Spring Break for many people, travel
May: School year ending soon, crunch time
June: School year ending, mega crunch time
July: Many people travel
August: School starts for many people
September: School starting for everyone
October: Ends with Halloween, very time consuming for many people
November: Ends with Thanksgiving, also crunch time for university
December: Christmas and other Dec holidays