They probably won't add a dot caster for the same reason dots have been reduced in the game since StB launch: there is a debuff cap on enemies that, once reached, they'll straight up start dodging status effects. Actually if you play WAR, have SGE, and both fc buffs up, you can reach that buff cap yourself in dsr phase 7 where you can dodge your own Bloodwhetting!

But yeah. Their design space regarding dots is hampered by server tech from the Jurassic period. That's probably why.