I think the real reason why Kaido conquered Wano is “Wano is the Ancient Kingdom”.
The story is close to ending. Oda has been hinting that Wano will play a big part with the Final Saga of One Piece. Already, we are aware that Wano is a country that was known as something different in the long distant past. The Void Century was from 900-800 years ago, and Wano was closed for 800 years. I think it’s not random.
Also, Clover has told us that the Ancient Kingdom passed down the Poneglyphs. But we are told that the Kozuki clan is the one who created them. It seems a simple case of adding 2 and 2.
There’s also the problem of Wano’s borders. It makes sense for the people of Wano to not want their borders open (even though this appears to be Kaido propaganda), but there is a logic to it.
Why would the World Government not want the country of Wano open?
Now again, looping in Clover: “The idea of the Ancient Kingdom is harmful to the World Government.”
– The Ancient Kingdom was not destroyed, but was forced into isolation. A deal was struck to preserve the people of Wano as long as they agreed never to leave. A crime on both sides. One crime to delve into the history of said Kingdom, another crime to leave said Kingdom. Cut off from the world, Wano lost its resources and influence and became insular, leaving it ripe to be overtaken by Kaido.
In other words, the Wano Kingdom Arc could also be described as “The Restoration of The Ancient Kingdom Arc,”, befitting of a battle that affects the fate of the world. Or it could be completely wrong, but that’s how theories be.