For me it’s simple – benefits.
I am based near heathrow and travel a lot throughout the world (US, Asia and eastern Europe are my main destinations). With heathrow as my local airport, BA flys to everywhere I need to go to and, in the vast majority, directly. So I can travel the world and collect miles.
If I were not to use BA, I’d have to have mileage programmes with, pretty much, every other programme out there. As a result, I would not have Gold (or even silver) on any of them, and my individual points would probably take me to nowhere inparicular. With one carrier, it’s all accumulated andd the potential benefits arrive sooner.
that said, having flown on most US-based airlines, a number of eastern European ones and some far eastern ones – BA is damn good. you can do a hell of a lot worse than having BA as your gateway to the rest of the world (just try American or Aeroflot)