And it couldn't run concurrent with the primary because.....
Because New York still lives in the Dark Ages when party bosses controlled everything they could grab onto. Heck, in this case, Patterson is clearly violating the state law on filling Congressional vacancies.
I don't think Paterson is violating the actual text of the relevant state law, which gives Paterson control over when and whether to schedule a special election. He may be violating the Constitution by refusing to schedule a special election, though.
Actually, I might have answered my own question, come to think of it. I'm not sure that under the relevant New York law, Paterson could schedule the election in September - but I'd have to double check.