Milliband, the leader before Corbyn, was also undermined by the zionists in the party because of his support for Palestine but couldn't use the "antisemite" tag because he's Jewish. Instead they labeled him "toxic" & said "Jewish donors were leaving the party in droves"

https://www.independent.co.uk/news/uk/politics/labour-funding-crisis-jewish-donors-drop-toxic-ed-miliband-9849299.html

So the answer is that unless labour party leaders swear alliegence to Israel, they will face the wrath of vicious and toxic zionists.

They definitely didnt help, nor did the right wing media or the Labour Party centrists undermining him

But ultimately he lost because of Brexit.

In his first election, despite the pressure against him, he took the Tories to a hung parliament and forced them to make a deal with the DUP. Cos people were sick of Austerity and liked his domestic platform

But when managing Brexit became the main issue in 2019(?), Johnson had a really strong message of 'oven-ready brexit', 'get it done', and Labour didn't have a coherent strategy. They didnt want to go full 'reverse it', cos lots of votes for Brexit came from Labour seats. They also didnt want to go full 'get out deal or no deal' because generally the left and progressive voters were anti-brexit.

Corbyn was elected to the leadership on the strength of his domestic and anti-austerity policies, and when the focus shifted to Brexit he was out of his comfort zone.

That's my analysis anyway. I liked Corbyn's foreign policy, but it wasn't what built his popularity