Because the reality is that it is them and us. When you chose to put your cross in the remain box, you made a conscious decision to say "I don't give a fig about the low-paid Britons who are finding their wages undercut by mass EU immigration. I'm all right Jack. The working class can kiss my arse". Unfortunately for you, a large number of the lower orders you so very clearly despise came out and made their voices heard.
This does need some sort of answer but it is hard to be both brief and polite. It's obviously wrong on many counts.
a) Many working class people did vote to remain. Equally many of those I guess Gutfright would classify as "them" voted to leave.
b) People voted to remain on the basis of many things. I can't think of one which relates to a conscious decision to stuff the working class. For instance concern about the long term future of Europe (which has been much derided but is a real fear). Europe has a long term history of wars, and in wars the working class suffers most.
c) The facts (or at least the best information I can get) is that immigration from the EU has reduced wages at the low end by about 1%. That's bad if you are directly affected but is offset by the increase in the minimum wage (I refuse to call it the living wage). However the real point is that it's about balance. The EU seems to have been good for the economy and that doesn't just benefit the rich - it creates jobs.
OK. So "Leave" won. I regret that, but I respect the decision. We can either go forward from here working out what to do next on a cooperative basis, or let the referendum create a split between whatever view of "us" and "them" you happen to have. That will benefit the right wing, who absolutely do not have the interest of the working class at heart.