UEFI gives you a much nicer initial environment — it's already in 64-bit long mode, for example, unlike BIOS which drops you into 8-bit real mode, with a maximum of one sector in which to perform all the gymnastics you need to do to load the rest of your program and switch to _at least_ 32-bit protected mode (notice how this PDF doesn't even go to 64-bit long mode).
UEFI also gives you better, easier to use boot facilities than BIOS: you can access a memory map, read from the boot file system, switch video modes, and even access network drivers. And you can access all this from long mode, unlike BIOS which only works in real mode. Then once you've booted you can take over the system by exiting the boot services, and have full control over the system just like you want for making an operating system.