i think alot of it is lack of practice and no emphasis on fundamentals.
often coaches are afraid to get players hurt practicing. so they dont hit during practices, they dont do hitting drills during practice. they dont learn how to tackle or take a hit.
this used to be a pro football thing, but they guys knew how to hit from pop warner, high school and college. then colleges stopped hitting during practice. then high schools.
now even pop warner teams to a certain extent have become pussified.
so you have kids at each level who have never learned how to properly tackle someone.
the shame of it is because they dont know how to properly tackle or take a hit when it happens in a game kids tend to get hurt more often negating any benefit to not doing the drills in practice.
for quite some time coaches on each of these levels have always felt the kids should know how to properly tackle when they get here. in major college often kids are great just based on sheer talent and dont have any idea about fundamentals. at some point this catches up to them and the ones who concentrate on fundamentals succeed, those who continue to try and do it on talent tend to fade.