I think it's very sad that the "out" camp has been largely commandeered by racism and xenophobia. This can leave a very bad taste in the mouth for the liberal left, and I think is making people go to "remain" by default. After all, who wants to vote with Nigel Farage?
But this is actually very strange. The EU is undemocratic and benefits the tax-dodging super-rich. It is developed to suit the needs of enormous corporations and banks. To vote "out" would be a very logical move for people with left-leaning politics.
IMHO we need to be very careful not to vote "in" simply because the other option has been taken over by right-wing shouty men obsessed with immigration.
I've posted a couple of links to reasonable arguments below (but there's loads of stuff out there if you look for it).
www.dissentmagazine.org/online_articles/left-case-brexit
www.independent.co.uk/voices/labour-mp-kate-hoey-why-leaving-the-eu-is-a-left-wing-move-a6687936.html