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?

[Aielyn]

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)

It's actually not just Nintendo games. Other games do it, depending on the game.

Want to have your mind blown somewhat? Call of Duty 3 for the Wii sold more units in America in 2010 than in 2006, 2008, or 2009, and only 20% less than 2007.

And what makes you think that Zelda is absent from that pattern? Twilight Princess sold over 370,000 units in 2012. A lot of games are lucky to sell that many units in their launch year, and it's still doing it 6 years later.