Unlimited immigration was great for the corporate low wage employers and politicians as it boosts the housing market and GDP generally and covers up The Feminist Careerists failure to reproduce natural citizens sufficiently.
Trouble is immigration of alien culture in sufficient numbers creates a threat to your culture and security.
Both the left and the right allowed the unchecked reliance upon immigration.
Its hard to fix once past a certain point but Britain is wrestling with it now as are/will all nations who allowed too many immigrants in for too long.