3 underlying stats that show the Cardinals fall from offensive prowess

Pitches per plate appearance

The early bird gets the worm, but the patient hitter gets a hit. Or something like that.

There are many benefits to a hitter seeing multiple pitches during an at-bat. Most coaches use either 6 or 8 pitches as the baseline for a "quality at bat". While it's unrealistic to expect every hitter to see that many pitches, having some who do see a lot of pitches is very beneficial.

Seeing more pitches in an at-bat wears a pitcher out, and it allows the batter to learn more about the pitcher himself. While more pitches won't necessarily lead to a hit, walks are more likely, thus increasing on-base percentage. Conversely, seeing more pitches increases the possibility of a strikeout. This is the double-edged sword of seeing more pitches.

While the gap appears to be microscopic year to year, the Cardinals' rank in the league has dropped dramatically. League average has largely stayed the same annually, but the Cardinals have slowly been dropping below league average.

Teams such as the New York Yankees (109 OPS+, 8th in baseball), Seattle Mariners(91 OPS+, 21st in baseball), and Los Angeles Angels (97 OPS+, 16th in baseball) have seen the most pitches per at bat, but that hasn't necessarily translated to on-field success. The Los Angeles Dodgers, the team with the highest OPS+ in all of baseball at 125 have seen the 8th-most pitches per plate appearance. The Miami Marlins rank last in OPS+ and pitches per plate appearance, for what it's worth.

Pitches per plate appearance does not directly correlate with success offensively. It is simply one cog in the larger machine that is offensive output.