I think the reason is because as shlomo hamelech said,
it’s better to be alive than dead b/c those alive can still do mitzvos, so the person thinks he’s not going to die any time soon so i can always do it later (It’s sort of like the paper that’s due on a certain date and you don’t start it until the night before) but regarding a niftar l’iluy nishmas there is a time frame so that gives him an extra motivation
I hope this helps