Mainly because of price, and with close relation to its performance (like its material and ability to not get damaged early). Nothing plastic though. I think that‘s what PVC is?
Copper is used because it lasts virtually forever and performs better than other alternatives. PEX is relatively new and you should check with your local building department to see if it meets code. It is plastic. CPVC is approved in some areas but is also plastic. Galvanized pipe used to be used many many years ago but it is not as good as copper and eventually rusts causing all kinds of problems. By the way, PVC stands for Poly vinyl chloride and is plastic used in plumbing and a lot of other things. CPVC is a special type of PVC that will handle hot water as well as cold. White PVC will not handle hot water.
Likely the switch or relay.
Usually worn out contacts in the switch.
I think I would rater eat copper than PVC. Most new homes have PVC pipes because of the cost of copper has gone up. I bet that copper would win for having less pipe damage with frozen pipes.
Rob is right. Pex is the new thing. Blue for cold and red for hot water. They are putting it in slab houses here and will withstand freezing and will expand. They are plastic and so is pvc. If you do not like the idea of plastic then you will have to go back with copper.
