I'm guessing it's all down to the passengers i know for sure say JUST FOR EXAMPLE if ANA started a flight to Japan say £700 rtn and TK done the same for £600 via Istanbul i would pay the extra £100 for the direct flight and same goes with Heathrow if they did the same flight £200 cheaper i would rather pay more to fly from BHX then travel all the way down to London by car or train and have all the hassel of getting to Heathrow from Euston or paying parking and dealing with all the queues at security.
And before anyone says "thats your opinion" yes i know but i'm sure i'm not the only who would do that.
(All prices and airlines were examples and not to be taken to heart lol)