Reality is a mixture of reasons. I would suggest poor pricing and poor marketing from BHX in recent years, happily this is changing for the better, it's got you talking about it after all. In addition the economic downturn affected the metro area of Birmingham badly, whilst London PLC was bailed out at any cost the lack of cash-flow coming from the banks crippled many businesses here. All of this has seen the airport struggle, that said it has maintained a set of connections and routes that no other uk regional airport can match. As has been said above it is also the best connected airport to uk transport links, the m6, m5, m42, m40 all converge on BHX as does the west coast mainline. No reason why BHX won't continue to grow now, the economy here is growing positively and forecast to remain growing, a smart traveller from north London would use BHX, as long as the marketing people keep spreading this news than I can see no harm in continuing to aim high.