I guess it's habit more than anything, that and collecting miles/status.
70% of my travel is between EDI and LON and given that my work is near Paddington, LHR is the obvious choice and BMI have a better timetable down to LHR as well as being considerably more generous with FF benefits than the competition (BA, AF and Easyjet).
I occaisionally have to travel to Pisa for work. I choose to endure 2 1/2 hours on a Ryanair flight. Not a decision based on price. Just I'd rather go straight there instead of hanging around in LON/FRA/AMS/CDG for 3 hours.
All I'm saying is there are many factors involved when choosing an airline and loyalty/status is just one of them.