I have been a UA 1k for about 10 years, despite living in London which is a BA and VS hub but has weak Star Alliance presence. Of course, BA has dozens of routes to the USA whereas UA only has seven, but I only ever fly to 2 of the ones UA has.
Why do I do this? Historically, UA was much more generous than BA. This year it won't be, so I will no longer be loyal to BA. Management don't seem to realise that, to attract non-captive customers you have to offer something more to them, but that's another story.