Then people would whine that it is too OP for PvP, so damage crits would be buffed, and heaven knows where that would go.
To be fair, crits already HAVE been buffed with all the stuff added in season 7. My engineer normally is already sitting at least 10% crit chance, and like...75% or so crit severity before equipment.
Anyways, healing is powerful already due to the simple fact you can 'spike heal' someone from 0 to full in moments. Not to mention the loads of passive heals and resists anyone can get now. So the game doesn't really need 'crit heals'.