I find it interesting that the vast majority of comments seem to focus on race and politics, and only one that I noticed that mentions social media and phones as possible factors.
I personally don't think we need to even try figure out the cause. We just need to not lower the standards for entry into subsequent grades, highschool and college, and the problem will fix itself without us figuring out "why" it's happening at all.
Not to defend anyone else's comments (which I actually haven't read through), but many of his have kids and see first-hand how poorly math is taught today.