Corbyn was someone who is politically a breath of fresh air for UK politics. Somebody who wasn't just another face on the same neo-liberal policies. The first genuinely left wing politician we've had in a major position since Thatcher destroyed the left at the ballot box. I voted for him in both Labour leadership elections because politically, I align well with him, I love the fact he proposes bizarre ideas like spending money on health, education and the environment, and I like the fact that he actually answered questions, honestly and in depth, giving thought to both sides of an issue and presenting a nuanced, balanced view. Unfortunately, that was seen as a weakness in the era of politicians who have to look like they know everything and bluster their way through interviews with soundbites, slogans and talking points. That was the first blow.
The second came when he faced questions about the Trident replacement and he gave an opinion that it was a waste of money, and also that as PM he could not foresee any circumstance in which he would use nuclear weapons. This got him painted as a dangerous leftie radical, and then he said that was his opinion not necessarily Labour policy, which got him called indecisive again.
Then came the anti-Semitism row within Labour which he utterly failed to get a grip on. And then, crucially, utterly failed to make it
look like he had a grip on it, floundering and flapping and dodging, the opposite of the straight talking people had elected him on. This led people to feel he was being evasive because he
agreed with the anti-Semitic wing of his party. Strike three.
Then, the final blow was Brexit. He was never that big a Europhile, the hard left have never been big on globalisation in general, and as such he was a lukewarm campaigner for Remain at best. He treated the whole thing like a parent reluctantly watching a kid's nativity - he felt he should be seen to do the expected thing, but his heart wasn't in it and he always looked slightly pained.
Once the referendum passed and the courts decided that Parliament must pass legislation to enable Article 50 to be triggered, he then made one of his only truly decisive leadership calls, and it was simultaneously the most bizarre one he could have made - he ordered his party to vote with the government, pass the bill, and trigger Brexit. And following the vote, sent round an email saying 'the fight starts now'.

Even now, I have no idea what Labour's position on Brexit actually is.
Two years at the head of the party have taken a politician who was honest, clever, thoughtful and actually straight talking
not as a euphemism for brash and intransigent, and turned him into a bumbling fool on the national stage. Given the opportunity to step up to the plate, he didn't. Whether that was the fault of the petulant children within the parliamentary party or not, the result is the same. He is not an electable PM, and Labour will be destroyed at this election.