Why? Oden was a conqueror, a Kozuki (the inventors of the Poneglyphs), Wano's strongest samurai, the Shogun's heir and also one of the few pirates who discovered the secrets of the Void Century and the One Piece itself. The Marines were wary of invading Wano essentially due to his name (they probably feared that he might've allied himself with Kaido; the panel below hints that if it were Kaido and Big Mom alone, Akainu would give the green light to Kizaru).
Do you think that the current Blackbeard (3.9 bil bounty) would be able to beat Kaido in an 1v1?