I recently installed Linux in an UEFI laptop and installed the GRUB loader wrong (MBR based). After this the system did not start in any OS. The boot device selector still listed the UEFI devices, but whatever was selected it did not boot. I tried recovery with Windows tools (fixMbr) but nothing helped out. Finally I solved by resetting the MBR (from within Linux recovery) and reset the protective flag. After this I could install GRUB "correct" registered as UEFI and I could select whatever UEFI device was available. What I am really interested in is why this happened and why the UEFI system denied to boot with this MBR not being correct. There must be a mechanism which compares the UEFI signature against the MBR. Is my assumption correct or might anything else explain this phenomenon?
Because BIOS use MBR partition style instead of GUID partition table that UEFI use.
Pay attention to article Legacy BIOS Issues with GPT:
Unfortunately, in practice, there are occasional BIOS/GPT quirks or even downright incompatibilities. Furthermore, many modern computers with EFIs provide a Compatibility Support Module (CSM), which enables them to boot using BIOS-mode boot loaders. The trouble is that the firmware may use the partition table type as a cue for what type of boot mode to use, thus causing problems when trying to boot in BIOS mode from a GPT disk. In either case, a computer with a GPT disk may not boot ...