Why would you use copper over CPVC or PEX?
Copper has a well-proven track record
Copper parts are more readily-available and it's easier to repair (eg, don't need the expensive crimping tool like with PEX)
Safer during fires since it is much more resistant to heat and giving off fumes 


That said, I am a huge fan of PEX. It's been around for quite a few years now (though obviously nowhere near as long as copper), and for the most part is proven. 


Pex is cheaper (in both material and labor costs), 
Pex is easier to install, with many less connections required (eg, try doing a long straight run through a wall or perpendicular through joists - PEX slides right in, copper requires a ton of little segmented pieces and couplers) 
Pex can look better when properly installed (when not properly secured, it sags, and because of the forgiving nature, you can have crooked lines and non-right angles).