Once they used E for Basic Economy, there were no more free letters left in the alphabet and W probably seemed the best fit at the time.

http://www.cwsi.net/delta.htm

DL has already switched to using PAG fare classes for code-shared VS Premium Economy flights. AF and AZ flights still use W fare class. Either way, Premium Economy can be identified in the fare code where the second to last character is a 'U'. C+ fares have a 'C' for the second to last character.