The trouble with variolation is that it requires advance planning to implement.
If you try variolation in the middle of a smallpox epidemic in the community, it may have very little effect, as people are infected and dying anyway.
It would only work if you variolate and then keep someone who is variolated from being concurrently infected by smallpox. That's hard to do.
To effectively variolate, you would have to reach an uninfected community or caste, and then try and do it before they epidemic reaches them.
If you try variolation in the middle of a smallpox epidemic in the community, it may have very little effect, as people are infected and dying anyway.
It would only work if you variolate and then keep someone who is variolated from being concurrently infected by smallpox. That's hard to do.
To effectively variolate, you would have to reach an uninfected community or caste, and then try and do it before they epidemic reaches them.