I would imagine its because the RBCs have other antigens/proteins on their surface (e.g. blood group types). NK cells kill when exposed to nonspecific signals and/OR an absence of class I MHC.
Although not necessarily related, it gets the concept across, RBCs have DAF which if is lacking will result in lysis of RBCs when the patient is sleeping due to complement. There are probably other mechanisms in place to prevent NK killing.