Because we're about to fight 2 Yonko head on and we won't see it until next year because we have to see Oden fighting a boar?
So! I don't see how that is a valid reason. If we don't know about Wano and Oden, then the war at Onigashima is nothing but a tournament. This flashback is gonna set the tone for the war.
If Oden fighting a boar gives us more of an insight into Oden, I am all for it. Besides, it is better to clear flashbacks now so that we don't have to see flashbacks interspersed with the fights.
Finally, OP fights are usually nothing great to speak of. Lucci vs. Luffy may have been an exception. Whereas OP flashbacks have always been great.