> Maybe cost is too high for the average customer?

Well yes, and even for the non-average one. Plus the goal is generally to build things which barely stand, a building which can face the vagaries of time for thousands of years would generally be way over-engineered.

Countries where pozzolanic ash is readily available do use it but that’s not usually the case. Fly ash is also being explored for that role.

An other issue is understanding of the material’s properties over time.

Also our modern world makes a lot of concrete.