It is a wise and beautiful thing to allow people of many kinds and origins to enter and become part of our country in search of a better life. It is a wise and prudent thing to regulate the inflow of new residents to make sure that terrorists are kept out and that the newcomers are brought into our institutions.
Why is it so difficult to say both of these things at once? For two main reasons.
Because on one side, demagogues curry favor with crude nativists who want to keep Those People on the other side of the fence. The Left is correct to say this.
Because on the other side, the party of the State angles to enlarge the underclass of clients whom it happily keeps in permanent dependence on itself. The Right is correct to say this.