That's because in 1970 the citizenship of this country was still 90% White. One super-majority tribe and one very minor tribe is very easy to contain. Plus we all know the history of that containment, so we need not go there.
Now you have one barely holding majority tribe, two major-minority tribes, and 3 or 4 more minor-minority tribes.
Our culture that contained the tribalism was nothing particular to our nature, just a result of workable demographics.
But yes, the Left is responsible for our current demographic shift causing strife.
Don't think though that they all secretly have cynical plans and want to see tribal violence. Most sincerely believe that human nature can be altered (it was known in earlier times as "New Socialist Man".
That's what is now one of the most fundamental determining factors between what is left and what is conservative. Conservatives accept human nature for what it is and work for the best within those confines.
The left believes in unlimited malleability.
Maybe these poor white kids were simply acting out in frustration over seeing the incredible amount of black on white crime being ignored by the Law and the media....maybe....
I suspect you and I have differing definitions of “tribe”. There are many tribes in the US, IMO; I don’t think it breaks down on just racial lines.