Copy and paste of my post from a similar question a while back...

After being constantly obliterated in the elections, Labour was declared unelectable. A new young leader came forward: Tony Blair and proposed that the party re-brand itself as "New Labour". They aimed to be 'socially left', but accept the superiority of the free market and adopt many Tory economic policies. Blair had a huge victory winning over middle ground swing voters and 'New' Labour was in power until 2010.

Since then they've been beat by the Tories twice, and many people began to say they weren't even a left wing party anymore - simply just a lighter version of the Tories. Corbyn, an 'old Labour', hardline outsider, put forward a return back to their true left roots campaign in his leadership bid and pledged to undo the so called damage of New Labour.

He won through a massive youth vote and support of the trade unions, but faced massive opposition immediately by the Labour MPs (nearly all were part of the 'New' Labour generation). They saw Corbyn as never being electable in the eye's of the general public and undecided swing voters.

tl;dr: you need to be vaguely 'centre' to win swing voters - he's seen as too left