This is just a guess, but maybe they already thought of that. Maybe it's technically possible at this point. If so, why wouldn't they do that?
#1. Speed running the actual os off sd is too slow. There's a reason we use ssd for boot drives and mechanical for bulk data.
#2. SD cards are an unknown, while NAND is a fixed variable. I.e., the NAND is always setup a certain way (partitions, filesystem, etc.), and you can trust that if it says there's 15GB free, there's actually 15GB free. With an SD card, what would happen if it says 100GB free but only has 1GB (fake SD card)? I'm guessing it'd require more coding and testing.
#3. I don't know about the rest of you, but one of the first things I'd try if emunand was stored on sd would be to run an emunand from a different switch. It's possible that running a different switch's emunand at this point would cause bad things to happen.
No idea if any of the above are true, just saying that there are some possible reasons emunand was initially released this way other than "they r stupid lol!"