The simple answer is that from a certain perspective Corbyn, and what people perceive as ‘hard’ left politics in general, appears to be for ideological causes first rather than people. You’re right that he’s spent his whole life campaigning for people against oppressive systems but he’s done it very much through the prism of structured left wing ideology which a significant block of voters are inherently skeptical about. This is why RLB is trying the Patriotic Socialism/Aspirational Socialism line - because socialism alone does not engage these people, though there are significant problems with this approach too.
To be clear, it is absolutely unfair that JC is viewed the way that he is and Nandy could be a lot more reasonable about it but to do so undermines the bounce that his successor gets or her value as a conduit to those voters that distrusted Corbyn. There’s a reason the Tories eat every former leader immediately after they sack them and only deify them later - because there is no strategic advantage in being nice to someone who failed.