So it's widely known as a fact that many Nintendo games have ridiculous legs. But here's the question that no one really bother to ask...why is this so?

[S.T.A.G.E.]

Otakumegane said:

 What gives Nintendo games such massively drawn out sales periods and 8year long shelf lives?

And why are certain Nintendo franchises absent from these patterns? (Zelda,Fire Emblem)

Because in this day and age they tend not to attract casuals as much as Mario. That and third party titles dont sell as well as exclusives on Nintendo consoles.