Hypothesis #1: the crackdown on defensive holding/DPI ever since the Colts/Pats clashes never really extended into the postseason. So we have effectively two slightly different rulesets, one for the regular season and one for the playoffs. Different ruleset = different game = regular season is less of an indicator of postseason success now than it ever was.
That's a pretty good point. Offensive holding is basically not called now in the playoffs, so teams with bad offensive lines (Giants, Cardinals, Steelers) that were just able to make the playoffs are more effective than they were in the regular season