the pod is so you can't change your mind once you press "on"

If you are alone in an apartment you wouldn't want to accidentally flood your whole apartment with nitrogen after you die, it could be dangerous to anyone who finds you. Similarly it wouldn't be good to release nitrogen everywhere with neighbors so close.

While a scuba mask is doable, other articles say your loved ones are in the room. So it's a way to say goodbyes.

Just a guess but I think not having any medical gear strapped to my face would feel more comfortable.