You hit the right note when you mention adhering to our age-old principles.
Many immigrants from Europe were looked down upon, in the early 20th century. But our society expected assimilation, and it got assimilation. Does anyone today really hate the Irish? The Italians? The Polish? They are just as American as anyone else and I think there is little to no lingering prejudice.
But blacks have been in this country a lot longer and have assimilated a lot less. Indeed, it is very much part of current black culture that they will not assimilate. They do not desire it. Their culture is too important to them, and (many of them) will not adopt mainstream American values.
This is what holds them back. They are given a chance -- but they will not take it.
But our society expected assimilation, and it got assimilation.
I respectfully submit to you that, if given a chance, the Irish, the Jews and the Italians would still be speaking their respective languages. Every people with history of any length respects and cherishes its traditions. What the insistence on American (in fact, Anglo-Saxon) standards achieved was that ethnic values were driven into the home: in any public place --- a shop, city square, on the job --- you were supposed to behave in a traditional (Anglo-Saxon) way. It's not that Jews, Irish and Italians were given a chance, and many first-generation immigrants have never abandoned their old-country ways. But their children, who spoke American English without an accent were given a clear signal: assimilate or you will not amount to much.
That is the only difference that I can see. Today, not only does the Anglo-Saxon majority fail to insist on assimilation but it actually celebrates and encourages whatever third-world traditions minorities do retain. A Yale professor with an Anglo-Saxon nane, when rejecting a $30M for a Center for Western Civilization, said, to wit: "There is nothing to study here: history of Western civilization is a history of war, pillage and rape." That's the essence of it. Western civilization is dying from self-hate.
People are rational. Blacks, just like everybody else responds to signals. The signal they hear since at least 1970s is "Savagery is good, Western Civilization is bad." They do what they are asked to do -- just like Muslims in this country, Britain and France.
P.S. I find it extremely unfair to both the truth and people involved when you say:
But blacks have been in this country a lot longer and have assimilated a lot less.
Had it escaped your attentions that they were the only group that came here involuntarily, and the only group that was subsequently enslaved. You expect a Duke living in a palace and a prisoner in his dungeon to evolve along similar paths?
Has is escaped your attention that a Black could not even use the same toilet as Whites just two generations ago? In sum: whatever chance at freedom they have was give fully only two generations ago; arguments about "being here longer" do not apply. And, you cannot blame a minority for the lack of assimilation when the majority destroys the traditrional goal of assimilation. Anglo-Saxons no longer behave in Anglo-Saxon ways; there is nothing for Blacks to which to assimilate.