Heres an interesting observation from personal experience last week.
Manchester Airport to Junc 26 (Bradford exit) 45 minutes
Same Junction to Leeds BRADFORD Airport 50 minutes !!
Point being ?
What an A*se of a place to get to !
I do not disagree that the journey west from this most south-westerly part of the Bradford conurbtion to LBA isn't the best. However junction 26 is almost as far from LBA as you can get whilst remaining in the borough. The other issue to note is that the great swathe of the Leeds-Bradford conurbation's affluent areas tend to be north of the cities. Certainly getting to Manchester Airport from many of these areas require that slog across either Bradford (or Leeds) that you allude to before reaching the motorway network.
Then of course there is the great variable in a journey to Ringway...the M62. Some days, 45 mins, others 2 hours? If you are catching a flight you legislate for the latter.
There will always be people that find MAN easier from within West Yorks and others that find LBA easier and I reckon Parkin, Halwood etal still have enough of the latter that curently leak to MAN to grow the LBA business.