The most obvious reason I see is that foreign labor cannot vote while domestic can. Since both compete for the same jobs the politician supporting foreign labor

The most obvious reason I see is that foreign labor cannot vote while domestic can. Since both compete for the same jobs the politician supporting foreign labor is risking losing votes. The more expensive is the labor the less votes it represents and politicians can get away with supporting "highly specialized" foreign labor as the money they get from employers of that labor outweigh the votes they lose from the domestic highly specialized workers displaced by the foreigners.