Interesting question, especially because it´s prooved for at least one nitride ceramics. Maybe it's due to their thermal stability on atmosphere. Keep in mind, that the temperatures for RAB reaches over 900°C. But e. g. for often employed silicon carbide it can not be the reason (it's stable up to 800°C and above). Another reason may be, that carbide ceramics form passive surface layers and this hinder the RAB.