Part of the reason why is that he's so prolific. (He's also Canadian and a science fiction writer, though you can't really hold that, or any social maladjustment stemming from that, against him.)
What's more, he stands upon a very big soapbox and he knows it. Though he doesn't appear to play to his audience: Even if he didn't have any readers, he would probably still write about the same idiosyncratic, self-indulgent crap he does now. So I would call him uncompromising in that respect.
Similarly, he's a foaming, humorless zealot on many issues, and he's an inveterate self-promoter. So he's definitely in that "love him, hate him, or love to hate him" category.
In fact, he's kind of like Howard Stern.
I also think most of the Cory Doctorow hatred comes from unalloyed jealousy -- given the chance, who wouldn't love to have a loyal following of millions of eyeballs on which to inflict whatever inane blog musings they could come up with?
(Attention is kind of like the One Ring: "Sure, those other guys were corrupted by all that power... but if I had it, I could do some really cool things.")
Personally, I find his schtick annoying -- in his BoingBoing role, you could gin up a Random Cory Doctorow Post Generator, and I'm pretty sure it would pass the Turing Test (in that it would be indistinguishable from the human).
Notable in His Absence
Anyway, Cory Doctorow is on paternity leave right now, which means BoingBoing is free of his usual flood of posts about knee-jerk anti-DRM/copyright absolutism, steampunk, Canadia, his latest book translation or XKCD mention of him, Disney-fanboyism, etc.
Oddly, to those of us who thought BoingBoing might suck less in his absence, the exact opposite has occurred: BoingBoing seems to suck more now.
Partly, it's due to the sheer decline in output. I haven't bothered to crunch the numbers, but the daily post count looks to be about 5-8 posts lower than before he went on his paternity hiatus.
However, volume alone doesn't explain it. My theory: In his absence, it's clear that the role of Cory Doctorow and his inane posts at BoingBoing is to make everyone else's posts look better by comparison.
I'm not sure what conclusion to draw from this.