I don't understand why with something like Mario Kart lots of people are like, "I'll buy this later on", when the majority of sales for Skyward Sword come from the the first month or so. I don't get what makes games more front-loaded. If someone knows, explain!

That's just how the market reacts ...

No more needs to be said about that ...

The reason why a game like mario kart has such good legs is because it's a one in a generation type of game. The people who own the system will buy it when it comes out and then the people who buy the system after mario kart comes out buy it as well. Since mario kart games come out only once per system, they don't have competition from other mario kart games. For a game like call of duty, the legs get cut short because one comes out every year. Also, skyward sword wasn't that front loaded. It's just massively undertracked here.

Some games just have more dedicated fanbases. For example, JRPGs like Final Fantasy or Xenoblade will have most of their sales in the first month or so, and that's because most people that want the game will get it right then and there.

On the other hand, there are certain games like Mario Kart or Minecraft that are more likely to be picked up at any given time due to their accessibility and longevity (Mario Kart is once per console gen, there is only one Minecraft).

Basically, target audience, genre and competition are major factors in the legs of a game.

Nintendo said that Skyward Sword sold 3.42 million in December 2011 and I think something over 3.7 million as of March 31, 2015. That's extremely front-loaded.