Corbyn in 2017 represented hope for change. People thought he stood for challenging the Tories. It was the hope that motivated voters and mobilised them
He was percieved as indecisive and didn't really stand up against the Tories. He always failed to go for the obvious challenges against May when presented with them. He was like the striker taking a penalty and bottling it.
He had given remainers the impression he was on their side and he'd given leavers the impression he wanted to leave. But the lack of strategy and the late move to a half arse support for a ref pleased no one. So he lost support in both directions.
He couldn't pull the trick of going for the hope vote twice.
Meanwhile the Cons couldn't lose support in both directions due to Farage's strategic move. (Remember ukip didn't field candidates in a lot of seats too)
Thus huge losses in seats the brexit party stood - through votes for brexit party and labour not turning out. And losses in remain areas to the Lds.
And the Cons haven't been squeezed in the same way and Johnson still in something of a honeymoon period.
Corbyn didn't change, you are right. He should have. Instead he was a one trick pony who couldn't adapt for a second election and looked on the backfoot and lost control of the narrative. He didn't inspire a second time.
He should have gone after the last election, but the loss was viewed as a victory by his supporters and activists without thought about how he could extend his reach next time. This was ultimately the fatal flaw - flogging a dead horse. It was obvious he didn't have any further to get further traction. Those approval ratings were a bit of a hint, but the party was too caught in its own echo chamber and didn't want to face up to that reality. Instead the party looked inward and had an internal scrap over control of it. They should have been looking outward and taken criticism in good faith rather than merely being about internal factionalism.
Labour (and the LDs) have an issue with facing up to material reality in a few ways. Instead they have indulged fantasies every big as much as Johnson. Its just they lack self awareness of this.