A variety of reasons really. Just to name a few;
N is illegal pretty much everywhere unless you are working as vet or in the business of actually making the stuff (as far as I know, maybe some exceptions but I doubt many)
therefore people don't want to get a criminal conviction or a fine or sent to a psychiatric ward for trying to obtain it. Maybe all of these or some of these apply and I imagine each sentence when found in possession is different.
it's expensive and customs and LE make it hard to procure. Granted some have success. Im sure there are a fair few on here that have it. I do envy them. They are very lucky but again like many of us just because you have your method doesn't mean you will use it. SI is a powerful thing.
I think it also matters a lot on what method of payment and where you are geographically speaking.
I personally for reasons of my health struggle to fully comprehend Bitcoin and monero etc. So that could be a factor for some.
I'm wondering why there are not more stories about people taking N successfully on this forum and also in the media? It seems like there are quite a few SN accounts - even in the news, but hardly any about N, despite it being most people's preferred choice? Also a lot of people on here seem to have it but not use it?
Plus sn is also legal/controlled substance in a lot of places and yes it has its restrictions and laws regarding its sale and usage but it's not like trying to source a drug specifically designed in most cases to euthanise people and animals.
it's just easier to get sn, although it's harder now, at least it's getting that way.
who doesn't want the gold standard of ctb, heck even people who aren't suicidal are looking to die in their sleep and N is basically that. I don't understand why anyone wouldn't want a peaceful death (granted some don't, but that's a minority)