Who exactly does he appeal to though? There are lots of people on panel shows I don't like (Russell Howard for example) who get good reactions off the audience, so I can see some people like them. But Bishop nearly always fares quite badly on panel shows; often the audience don't really go for him either. I've only seen a bit of his stand-up, but he struggled.
Struggled in what sense? I can only assume you mean he struggled in terms of your enjoyment of him, which isn't relevant in the grand scheme.
I saw Bishop in the latest series of Skins, reprising a role from the previous series, before I knew who he was. So I'm guessing he has been earmarked for success for a while now.
Interestingly*, Mark 'Bez' Berry was in Skins the other week as a rickshaw runner named Rick Shaw and it wasn't until the credits that I actually twigged he wasn't John Bishop's character, or for that matter John Bishop.