The same reason for the exact vice versa... And same reason for every other brand loyalty.
A combination of Insecurity, elitism, wanting to be part of the 'in crowd', whilst 'supporting the under-dog'. Wanting to justify your purchase desicion by aggressively assininating every other company.
Heck we have had fine examples in this forum of certain posters allegiances being for one product they just bought, irrationally hating on others and then in only matter of months or weeks when they have something else their opinion is suddenly the polar opposite and the irrational hatred is then thrown at the once loved device / brand.
People are odd.
Those of us who like to try everything and appreciate the best and the worst from ALL companies and devices are very much in the minority.
I find this to be the same for other things too, fashion, music etc. People pidgeon hole themselves for the same reasons I posted above.
I've always been eclectic. Musically, food, fashion, literature and mobile devices/tech.