Guys are having to adjust to what is basically a different game than it was even a couple years ago, let alone when they initially learned how to play 15-20 years ago.
I'm actually pretty amazed that defenses have been as successful as they have at adjusting to what is a shrinking tackling target. Can't hit guys high, can't hit guys low, can't crack-back block, can't touch the QB, essentially, can't attempt to tackle a defenseless receiver...
This is a different game from the one we all grew up watching.
Not that I disagree but I also don't feel that's entirely it. Eagles for example have guys who simply don't wrap up. They throw shoulders going for big hit hoping to make sportscenter or jar bar loose. Something as simple as wrapping up seems like a lost art form today. Some guys just need pop Warner 101 on tackling.