Plumbing Question: Rigid PVC vs Flex PVC

I am trying to open a can of worms here because I just can't understand this. If rigid PVC is so superior to flexible PVC (when both are schedule 40) why do half of the builders in my area use flex PVC? They're not fly by night builders either, these are reputable builders that have been doing it a long time. In my area the soil is all clay. Termites/insects aren't problem, but you never know what pests or an infestation might show up.

I get that rigid PVC is the superior product for...

Plumbing Question: Rigid PVC vs Flex PVC