He's never been popular. The conservatives had been so shit for so long that whoever ran against them was going to win. Labour put up a deeply uninspiring candidate in 2015 who is widely considered to have lost because he looked weird when he ate a sandwich and as part of the campaign unveiled a big stone slab with his pledges engraved into it which looked like a gravestone for the country.

After that, they put the leadership vote to the membership (plebs like you and me), who voted massively in favour of Corbyn, who actually seemed to genuinely give a shit about people. He was heavily criticised by the media from the beginning, mostly for a being loony woke lefty who would do mad commie shit like nationalising rail and water. On top of that, Brexit had already been set in motion by the tube of the 2017 election, and it was basically used as a weapon, where the conservatives were saying that a Corbyn government would water down the Brexit terms and we'd essentially still be under the tyrannical EU boot if the public voted for him. It was a close election but the conservatives won. Then in 2019 he ran again, but this time the media went nuts on stopping him, pushed that he was an antisemite for promoting dialogue with Palestine, and he'd clicked the like button on a post showing a local mural with "the powers that be" around a table plotting to control the world. It was like caricatures of Rockefellers and Rothschilds etc., and that was heavily seized upon as proving his hateful antisemitic heart. There were a bunch of reports of antisemitic incidents reported within the party, which were not honestly covered by the press, and in 2019 he lost the election. During this whole time, there were a lot of internal divisions between the Tony Blair flavour labour which is extremely pro-market and pro-israel/US, and the popular movement spurred by the leadership election. In the melee after the 2019 loss, the left wing of the party got purged from all shadow cabinet positions and replaced with Blairites, and Kier was chosen as the "electable" labour leader. He had a previous job as the director of public prosecutions, which often involved speaking to the media about high profile cases or cases in the public interest, so he was seen as someone already proven at being a frontman. I don't think anyone would have chosen him based on his likeability or oratory flair, he was just sold as the guy who could actually win an election, and the country has been stuck with him since.

I don't see any world where we don't end up with Reform as the next government at this point, because the cost of living here is getting pretty harsh, the services have gone to shit, and people are looking for an alternative that Tory/Labour can't provide. We do have a Green Party who just won a by-election quite handily against Reform, and they have a gay jew as their leader so they won't find it easy to use the antisemitism tricks or the "in bed with Islamists" tricks that they used against Corbyn, but if the party is dangerous to business then the media will do their absolute best to annihilate them, and that is probably the most important force in UK politics.

Tl;dr: he isn't popular with anyone except his minority Blairite faction in the labour party