The reason labour didn't put up a "brexiteer" for Hartlepool, is because that battle was lost years ago, it was unwinnable and had even less chance of being won by going further left (as the far right hate this), they had to go with someone more central or a remainer and basically, just take it on the chin (for now). Then when it goes to rat$hit they can say "look, I told you so, but you went and voted tory, and now they've done you", and "I had your best interests at heart", rather than some labour prop going "fcuk the tories, but yeah, brexit is actually great, we're going to get loads of unicorns and gold, even more whilst the tories are in control of the country".
Also, where are you going to find a Labour MP that thought and now thinks brexit is a good idea? Generally, they are quite well informed about what is best for their core voters/ area, but just because some of their constituency voted leave, it doesn't mean they were right to, and labour MP's voted remain by about 20:1.
Brexit is done, we're out, fake (or real) ****** about it and waving a union jack won't make any difference, and if you thought it was going to fail before, then you surely think it will still fail, and it will. So basically just wait for it to fail and then be on the side that's likely to be correct in 2-3 years when the GE comes. That's better than going down with the ship, and being wrong, as in that case you still won't get elected, yet you would have been proven wrong as well, so come the GE you have even less chance.
If labour start beating the drum that brexit is good for the country, and working class, then they're either lying, or wrong. I'd rather they didn't win and stick to the principles of what we can expect to happen in the next few years, and have more chance at a GE.