There were many new members of the party who signed up under Corbyn thinking that his left wing stance would have a massive appeal that could only grow and it was inevitable that he would end up winning an election, the close defeat to May supported to that to an extent. Within the labour left bubble he was nailed on to win the next election, that in part is why the manifesto was so bad - they believed the prior version didn't go far enough.
Any one who dared suggest that JC was unpopular and his approach to extreme was labelled a tory, or worse, blairite scum. It was pretty clear though out the campaign he was a big problem and his people were problems (the likes of Abbot and Gardiner where hugely unpopular). The result was a wake up call in that they had denied this for months in the lead up - the election confirmed it.