The problem, according to Microsoft, stems from the Xbox One disc slot. While the Xbox 360's disc tray was capable of either horizontal or vertical use, the new slot was designed with horizontal configurations in mind.
"We don't support vertical orientation; do it at your own risk," said Albert Panello, senior director of product management for Microsoft's Xbox division. "It wouldn't be a cooling problem, we just didn't design the drive for vertical. Because it's a slot loading drive, we just didn't design it for both."