A very valid point
The answer to this is the Labour Party buying the immigrant vote and as you know they extended this by throwing open the floodgates to mass uncontrolled immigration thus having millions of future voters beholden to them.
I'm not really sure how many years it's been going on so it might be wrong to say the Tories haven't had the balls to change it - however I suspect they don't have them otherwise Human Rights legislation would be binned by now.