***IMPORTANT UPDATE***
Korra will no longer be airing on Nickelodeon, but is however moving to some form of digital distribution. From what I've seen/heard, you should still be able to watch it on the website, as well as other sites such as Hulu (I don't remember reading anything about Netflix though). The reason for this is because of poor viewership, or so I've heard. More information will be revealed at Comic Con.
American politics are the reason behind this. Even if you live in another country, what goes in America with media affects the whole world because of a law passed here in the 90s.
People should know that when you are told a show has been canceled or removed from a network because of low viewership, that TV network is lying to you. They have another reason. Maybe there was a personal disagreement, maybe someone in charge there just doesn't like it, maybe they want to put a cheaper show up in their place. Viewership is never a valid reason and I'm pretty sure they know it. With standard TV, despite the prevalence of digital TV, the only way for networks to know how many people watched an episode, is through the Nielsen ratings. What this is, is a select group of volunteers that have their viewing habits monitored with equipment installed in their homes, connected to their TVs. It's relatively small number of people. They then take the percentages of people who watched something for at least half of the length, and project that out to the entire population. This cannot be rigorously applied in this way and the results they end up mean nothing. An episode that actually got 20 million viewers might have only got a Nielsen rating of 3 million viewers. And that might be only 9 people out of a 1000 test group that watched it (not real numbers but it is only a few thousand, not the entire US population). So cited viewership by TV networks do not mean anything at all, and they do not reflect what people are actually watching at all. They might as well be talking about their astrology horoscope for all the relevance it has on reality.
With online services like Netflix, Hulu, and Amazon, which all have their own exclusive shows, they instead can directly tell what is being watched by how many people. They don't have the typical excuses that TV networks have for canceling shows. They're all companies founded in the last two decades and they're much more modern. And they don't really cancel shows. Effectively shows on their systems can only be canceled by producers themselves and not the distributor. The law that was passed in 1996, called the Telecommunications Act of 1996. Title 3 ("Cable Services") allowed for cross-media ownership. This is why when shows were not continued by their original network before 1996, they often moved to another network. And after 1996, the show is just canceled. In order for a show to move somewhere else now, the broadcast rights have to be purchased from the original network. It's also why when an American TV network cancels a show, it's canceled for everyone, worldwide, even if the show is made in another country. For example, Farscape was a New Zealand science fiction show, but when the American Sci-Fi Channel (now Syfy) canceled it, no one in the world was allowed to air new episodes so it was canceled worldwide.
TV Networks got very corrupt and dysfunctional in the last two decades. You can see that most prominently in the fall of Syfy. They used to be one of the best TV networks and now are little more than a joke that's widely hated by former fans for senselessly canceling shows to replace them with wrestling and ghost hunting reality shows. The big three of new original online shows, Netflix, Hulu, and Amazon, are as I said more modern companies and they're more in line with what people want. All companies like to make money, but these ones, being modern, haven't lost their minds yet and understand that giving people shows they want to see, will make them money. They don't use the excuses that TV Networks use, timeslots don't apply to them, and they're generally more liked by content producers. As time goes on you'll see more and more new shows on these services. If Korra moves to one of them, then it's possible we can see the show continue for ten more years. No promises. But it's possible because it's not old, outdated, and out of touch like Nickelodeon and most other TV networks are.
Hopefully this little lecture has educated people and cheered them up about this.