Power consumption is an issue holding back drives from using the backside. This is really limiting, since the NAND on a one-sided drive needs to share that side of the PCB with the controller.
However, once you start talking about low-power/high-density drives, then we end up in the QLC territory, which is regarded as low-end. This makes it sort of a no-win situation.
Again, I have to wonder if M.2 didn't put the connector on the wrong edge. A connector on the long edge wouldn't have to span the entire PCB, like it does with DIMMs, but putting it there would've made it easier for M.2 drives to sit perpendicular to the motheboard and thereby alleviate the backside penalty.