Serial/Parallel/G1 use zero as the dirty card value because some architectures have a special zero-valued register that can be used to save a single instruction in the card-smashing barrier.

See also
"Urs Hölzle. A fast write barrier for generational garbage collectors. In Eliot Moss, Paul R. Wilson, and Benjamin Zorn, editors, OOPSLA/ECOOP '93 Workshop on Garbage Collection in Object-Oriented Systems, October 1993"
where this is exploited for SPARC.