Why? You remember what big mom values in her territories right? She values territories that MAKE HER FOOD. Her one and only tax felt protection is food. The reason she attacked multple islands was food for her wedding cake.
What exactly does Wano produce food wise that can be considered a specialty of the country that Big mom can't acquire elsewhere and have to tax the country?
See I don't even have to get into these this in detail. This arc is about Kaido. Plain and simple. The only person who can be considered a secondary antagonist is Orochi. Unfortunately even another yonkou in this arc doesn't change anything. Remember the one and only rule about one piece. PLOT BEATS EVERYTHING.
The plot of this arc is entirely centered around Kaido alone. The only plot point that big mom is involved in that isn't just because of Kaido is her amnesia plot with Tama. Everything else to do with her exists because Kaido is there. If Oda kills Kaido, you need NEW plot points to keep big mom there. And that's not happening.
Sorry if you haven't noticed these things. I've not got the energy to spell everything out for you.