i dont know if this'll even come close to answering the question, but afaict, its because most of the current labour party were given the roles they got by tony blair, who pushed the party rightward (branding it "new labour", and corbyn is one of the few people left from before blair took over.
the way electing a leader works is, they have to get so many MPs to nominate them, and then the membership (ie, anyone who pays £3 to join the party) can vote on which of the nominees gets to lead. corbyn only got nominated as the token left wing candidate, so they could point him out to say "hey we tried but i guess nobody wants left wing policies". but people who did want left wing politics back in the mainstream, joined en masse to vote for corbyn and he won by a landslide.
the rest of the party were pissed, and decided having an old man who doesnt talk in buzzwords and parrot watered-down conservative policies meant the party was now doomed and corbyn needed to be gotten rid of asap. every time a local election happened, theyd talk v loudly about how, if labour did lost this time, corbyn would be kicked out. iirc, labour consistently did better in each local election, but the rest of the party still tried to spin it as "yeah, but we'd have done even better if it weren't for corbyn".
they also realised that an investigation into the legality of the iraq was nearing its close, and corbyn was pushing very strongly for tony blair to be tried as a war criminal once the report came out. given that most of the current labour party owe their entire political careers to blair, and also voted in favour of the iraq war, they needed to make sure corbyn was removed from any sort of power by that point. some mps issued a vote of no confidence in corbyn, which passed, but now they've realised corbyn, as the incumbent leader, is entitled to run in the next leadership election without nomination, and given the support by labour members, is likely to win again, so the only way they can think of to get rid of him now is to get him to resign.
so most of his party is calling for his resignation. cameron, seeing a way to move the spotlight away from himself, also called for him to resign. the press, most of which is owned by rupert "fox news" murdoch, wants him to resign. the political editor of the bbc, who has a weird vendetta against corbyn, is stirring up as much shit as she can to get him to resign. most of the general labour membership doesn't want him to resign. the average person on the street doesnt give much of a shit either way they just want politicians to stop acting like children and keep us in / get us out of the eu already.
a bunch of labour mps started resigning from front-bench posts, one after the other on an hourly basis to keep the focus on how badly labour's doing and cause maximum embarassment to corbyn to force him to resign (meanwhile, the conservative party are also imploding and they're thankful for labour for keeping the media distracted). then they realised that corbyn was just refilling their positions with people who aren't going to stab him in the back at every opportunity.
anyway the new labour branch of the party are looking to split off into their own party, but keep the labour name.
its all kind of a mess and none of this even scratches the tip of the iceberg.