One-off capital ships carried the design process forward

Why were only one Neptune-class battleship and one Queen Mary-class battlecruiser built?

They were stepping-stone classes in a conservative Royal Navy process that introduced roughly three changes before moving to the next design.

A one-off ship can be the intended bridge between larger production groups. The Royal Navy's approach was conservative and iterative, with a limited set of changes embodied in each step. Building the ship allowed the service to move the design sequence forward without waiting for a perfect final form. Class size alone does not reveal whether a design was considered successful.