Because Chelsea had/have a higher % of working class fans than Arsenal or Spurs who often attracted the more nouveau fans. It was actually Arsenal that started it if I remember it correctly. In fairness Spurs fanbase is similar to ours in that they reach out to the Working Class Communities in North London, South London for us. Arsenal is pretty much all upper/middle class (match going fanbase that is) It's nothing offensive but just banter between us, West Ham, Spurs and Arsenal I.e Chelsea - Chavs Gooners/Arsenal - Goons West Ham/Hammers - Spammer Spurs - Spuds
There's a problem with changing one letter of Gooners/Goons to comedic effect - one will get you banned for use of racist language, the other makes you sound like a misogynistic twat.
More specifically, PNP, I think it's limited to posters on this and the JA606 forums. I don't know anyone in the real world that calls us spuds.