The Labour members were offered a choice of leaders.
The had to choose between a leader who promised to fight to expose the Tory Brexit lies, he argued we should not accept Brexit if it brings economic disaster. his argument never won over all those momentum supporters, they supported the candidate who argued we must respect the result of the referendum.
I think one thing to consider is that Labour members probably did not have Brexit in mind when they chose Corbyn over Smith.
I don't think that they made that choice because Corbyn demanded that we respect the referendum, or any other aspect of his Brexit position. And I don't think they rejected Smith due to his warnings about Tory Brexit lies and his wisely suggesting that the result be ignored if it was disadvantageous.
I'd say that at that point Besxit still seemed a lifetime away.
I guess they chose him because they believed he would genuinely introduce the many policies and changes that they believed were essential to rebuild this country and make it fairer and less nasty, whereas they didn't quite trust that Smith would do that - rightly or wrongly.
That's Corbyn's big selling point, I think. People really believe that he is crazy/eccentric enough to really revolutionise the way the country is run, and genuinely build a fairer Britain, instead of just saying he will, like everyone else does, but without delivering.
That he's a class A knobjockey in so many other ways seems not to matter compared to that. That's how desperate so many Labour voters in this country had become for fairness and change. On that score, I don't think I can really blame them, even if Corbs has turned out to be a nightmare over Brexit.
(Edited)