[Miller]

If I remember right, Dev chose not to make rotwood kits a thing due to the power of the Bamboo-bo and how that simple Ski scaling weapon when turned to rotwood is suddenly a very big casting tool for both priests and hexers. Unless Bamboo-bo gets a change I'd personally not want to see Hex/Priests get even stronger.

I'm echoing what Fern said essentially but, the only compatible damaging spells with Rotwood can just barely be counted on two hands. That being;

Eternal Darkness (Hexer)
Altera (Lantern Bearer)
Black Bolt (Void Assassin)
Detogate (Void Assassin)
Enma's Summon (Hexer)
Asura Fist (Boxer)

They're fairly split across several classes, with the largest amount of effective skills under the darkness domain being in Void Assassin. It'd be hard to justify this as a Hexer buff when a Rotwood Bamboo Bo would only serve as a stat stick for Enma's Summon.