to make it simple, the white ethnic groups, after WWII, were accepted by large by Anglo whites (the descendants of pilgrims all the blue blood white peeps who have been here for centuries). THis was done by accepting ethnic whites into the communities of Anglo whites which lead to them using the same facilities and mixing into a big ol pot of whiteness. THat's how white is white in America, besides 'weird' white people like Armenians, and Russians, and ethnic holidays like "St. Patricks Day".
Latinos, and Blacks on the other hand had been fighting for years to joininto the establishment, but to no avail. THey also were fighting for their own names. For example blacks have gone from Nigresses, negro, colored, black, African-American...THis whole time the establishment, white power structure, government whatever you wanna call it named blacks whatever they wanted to but still denied blacks the right of white Americans. ("Latinos" got thrown all over the place, they were hispanics, but classified as white, and mixed in with other latin groups, then the term Latino came about to put them into a benefecil political bloc). Anyway, the 60's, 70's came around and minorities said fuck that, we're proud of who we are, and we need some sort of authority over are lives and what we should be called viewed as.
So since the lives of many minorities are still not on the level of whites, and because of pride of their Ancestry you have a qualifier in front of the American title when ever a non-white minority is brought up.
