because RCS "just works". CVS doesn't. To speak with the words of Linus Torvals:
CVS sucks. Also, I'm really not good at doing this talking stuff. Oh, and all you guys at Google suck. If you use CVS, you suck. Everyone sucks but me. Yay, I wrote a Kernel! You suck! I am god! [mostly sic from a tech talk at Google]