A recent reference/comparison was made with Bristol/Lulsgate.
Well BRS's catchment area runs thru South Wales, up to around Gloucester, across to Swindon then down towards EXT and all the way down to Lands End and beyond.
All these people trekking up from Cornwall and Devon, LHR is a tiring drive away, LGW and STN and LTN etc. even worse and unless they dare to venture driving along the A303 they're all going to be driving along the M5 past BRS, the first 'realistic' international airport they pass by and 2 hours nearer to home than LHR, BRS has a mega catchment area.
On the other hand Kidlington is sandwiched between LHR, LTN, BHX and BRS not to mention others such as Gloucester, anything more than a 30 minute drive from Kidligton then far more convenient to drive to one of t'others which will offer better services, schedules etc.
UK is never going to have wall to wall airports, it doesn't need wall to wall airports, Gloucester, Kidlington and Coventry are just three trying to develop, they're all so close to each other and other airports also it simply isn't going to happen.