Much more emphasis on letting passengers inbound know about Manchester in the US is required.
Given that many Americans appear to drop geography, at least world geography in primary school, that may well be an uphill task. If MAN re-branded itself as "Edinburgh-South" or "London-North" they'd probably have more success. Thing is that stuff such as football (the real one not that rather strange North American imitation) won't be as big a pull as it is from the far east.