Mares are hardly an answer to the very information that you requested. The notion that left-wing political values and principles, centered around policies which advocate for a fairer society for all, are un-electable are patently false.
Clearly those policies found favour with a large percentage of voters.
What followed the 2017 GE, was a concerted campaign by Tories, right-wing internal factions of Labour, newspaper billionaire owners, other media outlets, along with smear campaigns that were directed both with foreign and domestic lobbyists, all because Corbyn was seen by his opponents, as being highly electable.
Yes, he was so highly electable he lost again May, who ran probably the worst ever election campaign by a Tory.
Yup, it was all a CONSPIRACY ( probably by those who eat bagels ), after all Corbyn was a really unlucky anti-racist who just happened to speak at conferences with antisemites, have a highly antisemitic brother, invite antisemites to HoC. How could he have known? He's so anti-racist he was devoting his time to fighting racism, researching who he was associating with would merely take too much of his valuable time down to allotment up.