My theory is pitchers throw too hard today, leaving batters little choice but to swing for the fences. It wasn't that long ago a 95 mph fastball was a big deal and put a guy in the tops of the league for velocity. Now it's commonplace, and no adjustments have been made to the diamond to account for it.
This also plays into why the shift is now, IMO, a completely unfair practice. The shift has always been around, but it was a lot easier for batters to pick their spots when the average pitcher was throwing 90 and below, as they did until maybe sometime in the 1980s. When betters are facing starters and relievers who give them half an eyeblink to decide to swing, it's not reasonable to expect much angling to happen. They're swinging and hoping for the best.
I'd be for moving the mound back, lowering it, or both. We would see most hitters come back to going for average.
Sluggers will be sluggers.