He managed to sway 3/4 of Labour voters as opposed to 1/4 of Tory voters.
He didn't sway anyone, anybody even remotely a follower of Corbyn understood he didn't stand by his principals as his stance and that of momentum would have told you.
Even if the recent reports of him actually voting out are bullshit, perceptions matter, as it did with pig gate, that on top of not appealing to his core voter base and a potential GE in the wings, labour need to move now.
There isn't some silent left wing majority that is waiting in the wings that will swoop them into power, the entryists have ruined labour and the crys of red Tory and blairites is what will kill the party for a long time, the country has been centrist for generations, grabbing votes from the greens isn't going to mean squat.
It's all well and good saying party members voted for him, when the general electorate think he is ineffective and wouldn't vote for him, poll after poll show this, it's not a media conspiracy, it's not a blairite coup, the man is ineffective and if he can't even keep that 'blairite' Watson on board then maybe the common factor is the man stuck playing student politics.