There are a bunch of factors here. Some of these were covered in the athletic football show and other pods. A lot of football writers have touched on them as well.
QB play is a big factor. QBs are skewing younger than ever before. Since the Brady and Brees generation retired there is a missing 5-6 years of QBs. From 2010-2015 there are hardly any QBs there who amounted to anything. With the 2010s came a new CBA. That CBA incentivized rookie QBs. There has been and will not be anything as valuable as a rookie QB performing on that rookie QB contract. QB contracts as a % of the cap have skyrocketed from 15% or so in the early to mid 2010s to now 20%+ for the top guys. But even the middle class of QBs is prohibitively expensive. If you have to pay, in many cases 15%+ of the cap for a mid tier guy you might decide to find a rookie instead. Because of that guys like Baker Mayfield, Geno Smith, and Sam Darnold aren't retained. Now to be fair most of us wouldn't have guessed these guys would take some huge steps forward, but teams are giving up on rookies quicker than they used to. The problem is that it can take QBs 5+ years to start to peak but no one has that kind of job security whether it is the QB, GM, or HC. And also many QBs won't have a Geno arc - a lot of guys just stay bad. So you don't necessarily know if a guy will make those massive steps forward or not and, again, decision makers don't have the job security to see it out anyway.
Defenses are making huge strides because the pendulum is swinging the other way. Since the death of the prevalence of Seattle style cover-3 we've seen an explosion in different styles of defenses but they all have a couple things in common. They major in disguise and they fuck with pass protection rules. Now, some of you might be thinking to yourselves, hey, didn't Mike Zimmer do that years ago with his casino blitzes? Yes, you're right, but now we are seeing those concepts on steroids. Defenses are also using more and more sim pressures, twists/stunts and line games, and, most importantly, are using clever zones behind it. Remember how when defenses blitzed there would be a hole where the blitzer came from? Now that hole is often filled with a different defender which absolutely fucks with your QB hots. Defenses also used to use a lot more man coverage behind blitzes but not anymore. Defenses are also spinning the dial and disguising coverages now more and more effectively than ever before. The answers to the test aren't nearly as clear before hand or even immediately post snap! It's a lot harder for QBs. KFP is 100% right that a lot of these are rudimentary coverages but defenses are getting to them in unconventional and unpredictable ways. They are also throwing more wrinkles in them like more inverted cover-2 "rain" where the corners and the safeties switch roles (a BB special). So the coverage structure might be conventional but the roles in it are unconventional! Again, more fuckery!
Quick edit re zone blitzes: not only do they take away your hot but now you have to find the new open area all while you are being pressured. Just nasty. Like others have said… before offenses were making defenses react and now defenses are taking it to the offenses.
Offensive line play is also impacted by a lack of practices and coaches haven't figured out the counter to all of these sim pressures, line games, and defenses attacking their pass protection rules.
There is going to be an offensive counter to these new defensive innovations. There always will be. We will see what happens! I love this so much. Things are just getting so sophisticated all around.
Rookie QBs have a hard job these days given all these circumstances. And college defenses can't hold a candle to the complexity of what NFL defenses are doing so it is a baptism by fire.
