Don't forget the writer of Sakazuki said this
Oda: I'd like to cherish Akainu
With that being said he's still waiting as the Fleet Admiral to get into action, Kaidou and Big Mom (Yonkos) came before Sakazuki could get put down by the protagonist
They got put down before Luffy could even fight Akainu
Imagine Akainu who's shown top tier performance, gets credit by the author and being an ultimate grudge opponent leaving a mark (scar) on the main protagonist being an easier obstacle than the previous antagonist