Earlier quoted context omitted.
Because Shingles is awful, for one. https://en.wikipedia.org/wiki/Shingles
Shingles and adult chickenpox are the reasons the UK National Health Service does not vaccinate against chickenpox. https://www.nhs.uk/common-health-questions/childrens-health/... They do vaccinate pregnant women who never got chickenpox and people with weakened immune system, though.
https://www.canada.ca/en/public-health/services/immunization...