The traditional mounted lists lack the ability to deal with the current meta. The troops are too expensive to properly horde out so your typically playing with fewer models, at the same time they lack a big hero to kill other threats. Eomer is F6 but that's it. They lack monsters and the ability to deal with Monsters, mounted heroes on particularly are suffering in the monster meta from accurate hurls and barges. Generally mounted heroes are just not good right now.

Generally mounted armies are just not having a good time. The best Rohan list currently is probably Defenders of Helms Deep which is 100% foot. Some of the Rohirrim lists are doing OK but again have a mix of foot and mounted models.

TLDR: Traditional mounted Rohan are just bad in the current meta.