Space. The 64gb micro already has little usable space for games, and hibernate requires that you write all of the contents in RAM to disk. Rather than fracture their feature set by model options, Valve instead decided not to bother with it (just guessing).

That, and as another person said, hibernate just hasn’t enjoyed great support under Linux. There are definitely other issues that need to be fixed, like the audio bug while docked and the need to disable half the CPU cores in order to have good emulation performance.