What I see is that offenses have become much better at dealing with complex schemes.
They work quickly, giving the defense less time to make changes and adjust to offensive formations. This strategy alone was so effective in the late 80s when the Bengals started using it that many teams just sat in their base defenses rather than try to make substitutions and adjustments. Can't get more simple than that. Now it seems like half the teams in the league go twice as fast as the Bengals or Bills ever did. That makes it really hard on the defense.
Also, they use spread formations, making it harder for defenses to disguise what they're doing and they use quick, short passes to minimize the effectiveness of complex blitzing.
So it's harder than ever for defenses to matchup with the offense, create pressure and disguise what they're doing.