My opinion is that Labour need to appeal to people who would not usually vote for them. You can call it the centre ground, you could call it Mickey Mouse, but the song remains the same.
You, Labour, can blame the media, other/ex Labour MPs, Trump, Putin, it matters not. As a floater, Corbyns stance on Brexit, and his general message just does not appeal, and the stuff that McDonnell comes out with fills an awful lot of similar minded folk with dread.
That will be obviously dismissed by the echo chamber, but in real life, Labour have to recognise the stark truth of it, not rubbish anyone who isnt a fully paid up member of the Corbyn/Mcdonnell view of the world.