It's because he's an aggressive dick who's "gotcha" passive-agressive Claim Chowder pieces are often disingenuously dishonest.
His habit of trawling for people's reaction to the very first iPhone ( the glorified feature phone with a 2G connection, no ability to do voice-and-data and no ability to write apps for it who's sales figures were basically a rounding error) and present them as if they are talking abut the latest juggernaut selling release of the iPhone with the justification that they should have seen what 10 iterations and 7 years of development was going to bring makes him an easy and satisfying target to skewer when he so grossly fucks up with his own opinions.
When I started reading Gruber, before I was even an Apple fan, I loved how he plowed through the anti-Apple BS that showed up so regularly on tech websites (and that I didn't even recognize as such before I started reading him). I think his style is a reaction to all the nerd bullying that Apple users have been subjected to over the years. (And really, you still see it everywhere in the tech sphere. "How's that one-button mouse?" "Mac gamers? LOL!" "iSheep" etc. Try asking a question related to Macs or OSX on any tech website: you'll get a hundred comments laughing at you for having bought a Mac. If you're into Apple products, it's hard to not grow angry and bitter after a while. Hence the endless flame wars between Apple and Android users on sites like The Verge.)
His habit of trawling for people's reaction to the very first iPhone ( the glorified feature phone with a 2G connection, no ability to do voice-and-data and no ability to write apps for it who's sales figures were basically a rounding error) and present them as if they are talking abut the latest juggernaut selling release of the iPhone with the justification that they should have seen what 10 iterations and 7 years of development was going to bring makes him an easy and satisfying target to skewer when he so grossly fucks up with his own opinions.