Marco thinks back to his days with Shirohige Pirates in a new flashback. Ace asks Shirohige to attack Kaidou since he has a promise to fulfilled. Izou and Marco explain that they only knew about Oden's death years after it happened. After that, they talked about attacking Wanokuni many times but stopped since they know what will happen if they attack Kaidou, there will be many sacrifices.
Ace asks Shirohige to let him go alone. Shirohige is angry saying Ace is ahead of himself thinking that he can defeat a man Oden can't win against. Then Shirohige beats Ace up.
I see alot of people already raging about this bcs it kinda destroy WB character I mean he's the guy who will rage war if someone did something to his family like he did with Ace.. but hear me out this is Marco POV not WB as we all know at this point of time WB already knew about One Piece and the Void Century.
I think the reason for WB not attacking Wano is simply just "there will be many sacrifices." but he did just that in MF many of his allies died just to save a single person but why didn't he do the same with Oden? and I think the answer for that is WB sees a bigger picture after learning everything from Roger he probably knows Oden has his own reason why didn't he ask for their help during that time and also why he didn't bother save Wano because of that big picture.
The reason why he save Ace while he ignore the situation in Wano is because WB believed during that time Ace is the "one" it could be he thinks Ace might be joyboy he is Roger's son the Pirate King the guy who found One Piece and he's also a D. so he did what he did in MF gave up on everything to save Ace but since Ace dies he says "someone will show up to carry Ace will someday that's how it has been passed down for generations."
So, yeah that's my take on why WB didn't avenge Oden not just because of there will be many sacrifices and Yonko don't attack each other because other yonkos might take advantage of that that's why they have been in deadlock for years as it was revealed in Ace novel.