He's actually a very bright man, I don't know why so many on the left buy into his 'act' of a loveable idiot.
He couldn't stand, one of two things would happen, he'd lose to the "anyone but Boris" candidate, then have it hanging over him that his party rejected him, or he'd win, have to deal with A50, or call a snap General Election, which he'd probably lose, or be in a coallision - and the Tories punish losers hard, it would be career over.
He just refuses to join in now, step away, make it all about Gove - who is unlikely to win the leadership contest. Let someone else sort out the mess stage, hope and pray that actually leaving the EU works out ok for the UK, by which point everyone will be ok about it.