The EXP and tomestone reward is proportional to the general time investment and cutscene-intensive nature of those duties.

Unless Thordan Normal gets actually rebalanced (item level sync like Endsinger, HP rescaling to counteract potency creep and perhaps giving the fight some mildly zesty mechanics or healing checks), it will never be part of this roulette. Similar but not to the same extreme this applies to Shinryu, Hades and Endsinger.

The fights are simply not nearly as long as the MSQ roulette duties and people would perhaps just hop out if they got the long duties (what people did with Orbonne/Nier raids vs CT raids).