It was necessary because the civil rights movement had become stronger and stronger, the previous way of dealing with immigration was based on the national-origins quota system stablished in the 1920's. In this way each nationality was assigned a quota based on it's representation in past U.S. census figures. As you can see, this tended to discrimination, specially towards Greeks, Poles, Portuguese and Italians.
As the civil right movement claimed that all human must have the same rigths, this had to be modified. The Immigration and Naturalization Act of 1965, abolished the previous quota system and stablished an immigration policy which main goals were reuniting immigrant families and attracting skilled labor to the U.S.