I think it’s because NaNo is over after the thirty days. A large number of people finish out their time at NaNo and never look back, never move forward. Those of us serious about writing know that manuscript not only has to be drafted (which is never done in 50,000 words), but then revised, proofed, revised again, etc.
It’s a much bigger commitment than 30 days, and those of us who spend time writing all year long don’t’ have time for the games and artifice of NaNo’s structure.
I remember reading on NaNo forums people who spent months in preparation, creating profiles of their characters, of detailing their settings and plot points, all so that they could begin writing on the stroke of midnight, and all I can think of now is that instead of doing nothing but preparing to write, you should write.