Why is a TTT diagram called isothermal transformation diagram?
They are two names for the same procedure.  A steel is heated to a temperature in the austenite range then quenched/quickly cooled to some specific temperature. Held at that constant ( isothermal) temperature ( T) for a specific time (T) then quenched to room temperature for examination of the transformation (T) products.  You may be thinking of CCT ; continuous cooling diagrams ; where a steel sample is austenitized  then continuously cooled at some specific rate.  Then examined for transformation products. The results can have similarities depending on alloy content , etc.