For some people, it may just be that they like one game and not the other!There is some psychology behind the either/or mentality. It's a little like the sunk cost fallacy: the individual spent money on one and now has to validate that choice to themself, defending the selection to avoid thinking that they wasted money.
I heard "there can be only one" from my spare room the other day with my D&D books and the sounds of clashing swords. I think I heard the sound of a head hitting the ground and then the lights exploded and lightning zig zagged around the house.