There are 02 reasons of using categorical data instead of ordinal data in Net promoter measurement :
1) ordinal data has more values specified while categorical data groups the parameters in a category thus reducing the values into a single category. This helps in quick comparison of the categories with one process to another process and also helps in taking actions on the bottom quartile measurement which is detractor easily.
2) apart from this, the categories are defined in regards to customer experience depending on the function.Thus, instead of working on 0 to 6 counts and taking action, the team works on Detractors or Red alerts(0-3) and frame actionables.