Two words; Ability bloat.

For RDM to properly function as a hybrid, (tanking this duty and healing the next, for example) they would need an above average amount of skills to cross class to simply function. Otherwise, nothing would work well enough to properly fill any roles.

As a tank, they would be gutted of defensive cooldowns to make room for DPS buffs. As a DPS, they would lose damaging skills for healing boosters. As a healer, they would lose certain heals for enmity skills, etc.