As an American, I'm just trying to learn
Why is Corbyn unpopular? The best analogy we have is Bernie Sanders, who remains respected and has the energy of the young voters.
He's "similar" to Bernie, and is popular with a lot of youth as well. American/British public are quite different though, and identity politics isn't as much a thing in the UK. Well, race/ethnicity identity politics anyway. Over here it's more about social class/wealth politics. The middle class/rich hating the poor. Even Brexit in amongst definite xenophobia is still often about rich vs poor. The perception that immigrants are all dirty poor beggers wanting handouts. Britain on average has a dreadful amount of cold-hearted, uncaring people lacking in compassion and being fueled by a Government which tries to unfairly shift so much blame onto the poor, disabled and less fortunate. They've even got the general public believing the poor/immigrants are the sole reason the NHS is failing.
Corbyn is just too divisive and at times a bit too selfish. There's only room for one party of selfishness and that's the Tories. Labour have flown too close to being Tory-lite too often. The UK, and largely England, need a better alternative to the Tories. It still warps the mind how many working class/poor vote Conservative in England, but one has to believe that is because they're being made to believe it's those who are even poorer than them, or the immigrants who are making them worse off. Labour aren't doing a good enough job of showing why compassion, care and positivity is a better alternative than hate, spite and aggression.
It's the same in germany. People essentially vote for Merkel and i'd bet the CDU will lose atleast 10-15% when Merkel quits.
We had some state elections yesterday and less than 50% knew who the leader of CDU Schleswig-Holstein was yet they gained and won the elections based on Merkels (yet again) rising popularity.
It's understandable to a certain degree, and it is why your leader does have to be objectively "good". Which is why Corbyn is tanking, he has many decent beliefs, but just isn't a leader. However, people are stupid as shit if they can't accept a political party is not one person and is a large collection of MPs with many policies. Even in America, it's not "just" the President who runs the country. Far from it.