It's just a personnel thing, probably. It's tempting to blame Mattingly or the coaches but at the end of the day it is more likely a combination of these two things:
The team being built around a few hitters who the organization thought were Power + Hit dudes who ended up aging into hitters with much less HR power than anticipated. This applies to Kirk and Bo, but also Vlad to some extent / from time-to-time.
The team being filled in with hitters who don't hit for power. Clement, Roden, Wagner, Gimenez, Straw, Lukes...
Those 2021 to 2023 teams featured guys like Chapman, Jansen, Semien, Teoscar, a younger George Springer. Simply put, the personnel had more pop.