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)

Lack of competition, lack of sequels.

IPs that are more popular among a more casual, less informed audience have better legs (e.g.: Gran Turismo).

Most Nintendo's IPs are like these, with some exceptions like Zelda, that have more core followers that know when the game is releasing, want to play it asap, etc.

the fan base is not the brightest they don't mind paying full price for a 5 year old game and since they stay full price for the entire gen most fans think they are new games, it's actually a very good strategy by Nintendo ^.^

This post may or may not have been moderated by axumblade.

Doesn't explain why games like Super Mario DS remake having better legs than most games even today.

My guess is that the evergreen titles appeal to a broad base that often includes that always underestimated "casual" segment. Every year some few million kids turns 8 and can now handle more advanced game mechanics. This segment doesn't care if Mario Galaxy is 2 years old or 5 years old. If it looks fun and appeals to both parents and kids it's gold.

Zelda is a bit above that level to where kids are picking their own games and are far more interested in games that are the new thing all their friends play. FE is much more for the high school and up crowd that would enjoy strategy and this demographic is likely to either get stuff when it releases or not at all

Huh?

Because it's Mario! When a person buys a new Nintendo system, they are going to pick up the Nintendo first party games like Smash. If you own a Wii, you own Smash.

This seems like the best answer so far.

Mario is really the only one. Pokemon, Zelda, Fire Emblem, etc are frontloaded