How come when you only like 1 company your dubbed a "mark"?

For example yesterday i asked a question comparing Sting to Undertaker and everyone called me a WWE mark.So what?I don't watch TNA it's not a big deal.They just don't appeal to me.It's the same thing with WWE fans.I have seen WWE fans call TNA fans TNA marks because they don't watch the WWE.Indy's fans are the worst.Gof forbid you don't watch Indys.