Each controller and subcomponent on the motherboard needs a driver that correctly puts it into low power and sleep states to get battery savings.

Most of those components are proprietary and don't use the standard drivers available in Linux kernel.

So someone needs to go and reverse engineer them, upstream the drivers and pray that Apple doesn't change them in next revision (which they did) or the whole process needs to start again.

In other words: get an actually Linux supported laptop for Linux.