Unfortunately for MAN those AMS/LHR connections and indeed local pride work against it !
The Yorkshire Post would i'm sure rather broadcast that they have a thrice daily connection to LHR than implore people to drive over to Manchester !
It is simply not seen as the major airport "For The North" as it was in the 80s.
The 'problem' with airports is that the economic benefits tend to be spread over a smaller area than the catchment area that generates the benefit. About 30% of MAN's traffic is from Gtr Manchester, but 80% of the jobs and 100% of the dividend stays within GM. So, if you are responsible for economic development in one of the other major cities in the North and you know that your own area generates some part of the other 70% of MAN's passenger boardings, then logic dictates that you will try and keep as many of the passengers locally . It's the same reason a few on here have such a 'thing' going on in respect of LHR; a sense of economic injustice that traffic generated in GM generates economic benefit in the South East.
In the case of MAN, had the powers that be contrived a way to share a sense of ownership across the North it might have been a different story. It seems obvious to say, but if you want it to be ' the airport for the North' there should have been some strategy in place to make it so. Some of the people I came in to contact with in the 90s were so dismissive of anywhere that wasn't MAN it was shocking.