Simple: it’s all about money.
I favour policies that make poor people richer (if necessary by taking money away from rich people). That means I am nearly always opposed to Cameron and Osborne, who favour policies that make rich people richer (if necessary by taking money away from poor people).
But Brexit is a policy that will make everyone in Britain poorer — both rich and poor. (Don’t take my word for it: see what the Bank of England, the IMF, the World Trade Organisation, the OECD and the World Bank say.)
So Cameron and Osborne oppose Brexit because it will make rich people poorer, and I oppose it because it will make poor people poorer.
Strange times make strange bedfellows.