One of the issues is that demand for bunks will be massively greater on overnight flights. If we imagine an airline which does routes which are predominantly east-west and back. Unless you can convert the configuration to no or few sleepers for the westbound, it's a non-starter. Alternatively we need a true RTW airline which could configure its eastbound fleet with bunks and its westbound fleet conventionally.

Oddly, SAA is one airline which operates a significant number of turns which are exclusively overnight. This could work for them, were they to want to be innovative.