Because that’s the only explanation that Steven J. Vaughan-Nichols sees in this. It can’t be to prevent, say, boot-sector malware from installing on the machine. Microsoft’s single sole solitary goal is to enslave the masses and keep them down on the Micro… excuse me the Mafis$oft plantation.
That said, I do agree with SJVN at the end. OEMs would not get on board with this, because it probably forecloses on them customizing the system with preboot software (which they’ve been playing with before).
That said, there’s also no evidence that OEMs won’t provide a switch that turns off secure-boot, so that if you must install Linux, you can still fiddle some setting in the UEFI to install it. And if you are using Linux, you must be used to fiddling your settings by now.