Its simple because Luffy doesn't beat Marine bosses. He never did. He fights them but he even make his underling Zoro finish Captain Morgan. Even that happened via underling.
- This series is about Luffy fighting Yonko and previously Shichibukai, not fighting Admirals. Oda said that. BB both was a Shichibukai and now is a Yonko. Akainu is none of them.
- Luffy said he will fight Admirals and Chinjao said the same but that's just because he has to because Admirals attack him. This series is not and will not about Luffy fighting Admirals.
Because there's a reason for that; Luffy finishing an Admiral accomplishes nothing for the series. Luffy doesn't want to be King of the world he wants to be PK and beating Admirals does nothing for that.
Revolutionaries wants to defeat Celestial Dragons and their no.1 protectors are Admirals. Dragon and Sabo need opponents, Luffy's underlings and his allies need opponents, Admirals will fight them.
Its Revolutionary job to defeat Admirals, not pirates. Or BB's goal if he wants to rule the world as King of the world, Luffy doesn't want that.
Akainu killing Ace has relevance to Sabo as much as to Luffy only that Sabo did nothing about it so he has to do more + he is a Revo. He has more reasons to fight Admirals and Akainu.
Only if Luffy follows WB's path then he needs to save someone from Marines still can do that without beating Akainu. He can defeat Marines with different methods. Marines are an organisation it doesn't depend on Akainu alone.