Could be for multiple reasons.
1. She's unaware of just how bad Wano is. She hasn't been on mainland Wano in 20 years.
2. Momo realises what he must do to open the borders and asks Yamao to assist him
3. She recognises that she doesn't have to literally be Oden and can have own personality
4. An unexpected 3rd party shows up. Both BB and the Marines have dropped hints of going to Wano. It will teach Yamao there are more threats to Wano than just Kaido
These are all just possible ideas, but Oda can come up with anything really since Yamao's not giving up her desire to adventure. She'll still get to do that with Momo.
1. She's unaware of just how bad Wano is. She hasn't been on mainland Wano in 20 years.
2. Momo realises what he must do to open the borders and asks Yamao to assist him
3. She recognises that she doesn't have to literally be Oden and can have own personality
4. An unexpected 3rd party shows up. Both BB and the Marines have dropped hints of going to Wano. It will teach Yamao there are more threats to Wano than just Kaido
These are all just possible ideas, but Oda can come up with anything really since Yamao's not giving up her desire to adventure. She'll still get to do that with Momo.
Also, why would the state of Wano make her stay? Everything will be fine once Kaido, Orochi and their poison factories are gone...