Not really sure how the Midlands is described to be in terminal decline, it has by a long stretch the 2nd largest economy in the UK, some 25% larger than the 3rd biggest economy, every reputable forecast suggests this will widen further over the next 20 years, indeed the forecast is for the South Coast to grow and become the 3rd largest economy.
The difference in markets between BHX and MAN is simple and it's geography, MAN is situated in the middle of 4 major urban conurbations, Merseyside, West Yorkshire, Greater Manchester and West Midlands. Add to this the significant other urban conurbations or Lancashire, Cheshire, Staffordshire and it's easy to see why the market for MAN is larger than of BHX.