I think there's a lot of truth in LGS 6753s reply to which I would add.
It would be interesting to study trends in premium (ie business) traffic from regionals. For example, whereas LBA has done well on bucket and spade,vfr and other such leisure markets, I suspect business traffic is down or maybe level over say 20 years. One reason for this is that rail has improved its offer so business traffic to destinations such as Paris and Brussels have switched, and Bristol, Edinburgh etc are no more. By improved offer I mean frequency, hours of operation, wi-fi enabled, not just speed.
Small airports -- excepting islands-- relied for a baseload on third level scheduled services which have now largely gone, and charters which have now largely gone to big brother down the road which offers more choice and a better value proposition.
The economy is centralising around a few growth areas -- places like Middlesbrough are sadly not what they were economically.
There's a pull factor too-- the condition of the housing market. Where might you build houses these days without getting massive grief?
The bloke from Birmingham Airport was pilloried for saying England needed about ten airports but actually he was probably about right.