It's difficult to fully express over text, but I'll try my best. Also it's different for each person I know, so don't take what I say as a unified answer. It's also in no particular order, just as they pop into my head.
Case in point would be brexit. (IIRC) The majority of Labour MPs supported remain, while the majority of working class voted leave.
Then you have Labour voters and MPs, attacking and labelling working class brexit voters as racist, bigoted and fascist. Often told that they are too stupid to think for themselves.
To clarify I voted remain (you may not believe me, but I did). But I know plenty of people who voted leave and they aren't any of those labels.
Less focus on helping the poor and more focus on getting rid of things like trident or the mythical wage gap.
More concerned with what's happening in Palestine, then our own country.
Little to no concern over the effects of mass migration and its effects on working class people.
Triggers broom is the best way to describe Corbyns Labour, 10 new handles, 4 new heads, but still the same broom. By this I mean towards the end of Blair, Labour was devoid of voters, so all the extreme left wingers joined to fill the void and now run the party. Can you still call it Labour when the communist party tells you to vote Labour?
You can see the effects of this by the rise of UKIP and Tommy Robinson. I'd heavily defend the idea that if Labour still represented the working class, you wouldn't have these groups.
We have a large proportion of the UK voter base with no home. Because of this they'll latch onto what ever group they can.
There are more reasons, but I have to cut it short.