Everyone seems to know exactly why, but he hasn’t said. My answer is…
I don’t know.
But speaking as a very minor Twitter celebrity who gets a tiny fraction of the attention Whedon gets, the more followers you have, the more Twitter becomes a firehose of hot steaming hatred. If you’re a feminist, multiply the volume by 10 (if a feminist woman, multiply it by 100). I’m at the threshold of what I can manage personally — I block a lot of people — and there are few days when I’ve checked in and found hundreds of raging argle-bargle blurts, and I just walk away for a day or two, until it dies down.
It doesn’t matter what you do. If you write something that a thousand people find wonderful, that just means there will be another thousand people who hate you for it.
The only way to ‘win’ at Twitter is to a) be so blandly inoffensive and uninteresting that no one cares about you, or b) be a fucking troll who shouts stupid things at people and gets insta-blocked by everyone except other fucking trolls. Those people find it a calm and quiet place where they can indulge themselves without interruption. People who are intelligently challenging or have complex positions that don’t fit neatly into boxes, like Whedon, are going to find Twitter to be a hell-hole.
So I wouldn’t pin the blame for driving Whedon away on any faction. Twitter social interactions simply do not scale well, and Whedon was at a level where the volume was deafening.
I’ll also say this: being a man and supporting feminism means constantly running the risk of stepping on land mines. It’s not as hard as being a woman, but we can’t possibly understand the experience of being a woman, and we can be constantly surprised and find that we’ve screwed up. It’s tiring, and there’s always the temptation of running back to the bar and joining the bros and not having to care about half the population of the world.
It’s terribly unfair, I know, that we men have the privilege of being able to step away from the minefield, but we also tend to be total klutzes at navigating it, so we get blown up now and then. I wouldn’t be surprised if Whedon wasn’t taking a break from the stress, because he can.
Also, because I’m still willing to blunder into the minefield, I’ve read a fair number of feminist perspectives on Whedon’s work, and some thoroughly hate him and think he’s an arrogant guy appropriating femininity to advance masculine perspectives, and others think he’s relatively enlightened, for a man. No one seems to think he’s the perfect paragon of flawless feminism. I like his work, because if nothing else, he makes his women characters interesting and complex and central to the story, even if they’re staggering down a few well-worn tropes (Dollhouse really bugged me, for instance: Stepford Femme Fatales).
For what it’s worth, I rather liked this analysis of Black Widow, Feminist Action Hero. But I can also see how she would trigger the Twitter Hose o’ Hate.
And of course, right after I post this, Whedon explains himself. I have the consolation of knowing I pretty much was exactly right.