M.2 cooling can be an issue when the bottom side of the PCB is populated due to the larger capacities. Cooling should be a lesser issue in general because all the NAND is not active at the same time.I agree. Perhaps 8 TB is pushing the limits of current technology? I imagine it's easier to manage cooling, reliability and so on in the 3.5/2.5/EDSFF formats.
In the server space M.2 are more common as boot drives where I suppose they see a lot less activity than their carrier mounted friends who are fighting with database loads and whatnot 24/7.