I think that is likely because the two states have different levels of confidence, similar to how "not guilty" is not the same state as "innocent". It is relatively easy to identify unsafe, especially when negative effects arise quickly. It is harder to reliably determine safe as it may just be a matter of time before negative effects arise.

For people who won't accept the vaccine regardless, it doesn't matter either way. For people who can be convinced, I think that obvious evidence of caution may be helpful in convincing them.