Perhaps when Kevin Drum has figured out why Southern Californians insist on giving highway numbers the definite article (as in “Take the 10 East to the 405 North”) he can work on the much more significant question of why San Franciscans give neighborhood names the definite article (“the Castro,” “the Mission,” “the Haight,” “the Tenderloin”).
I gather that the understood missing word is “district;” “in the Mission District” would be more natural than simply “in Mission District.”
Maybe there’s some sort of clue to be found in the exceptions: as far as I know, it’s always “Pacific Heights,” “North Beach,” “Russian Hill,” “Nob Hill,” and “Noe Valley,” without the articles. Is that because they name geographic features? Or is the rule that one-word names get the definite article but two-word names don’t?