So the harmonic convergence is weeks away... the once in 10 000 years event is happening a few years away and no one told Korra. When Aang gave her back her powers, he didn't think to mention:"So I have something very important to tell you that I've been trying to say for 16 years it has something to do with the avatar's mandatory participation in contest that decides the fate of the world, also it's in 7 months."
Whats worst is apparently humans in the real world knew about it too, the fire sage told her only by the virtue that Korra randomly happened to wash up on the island. No one thought it would be a good idea to tell the avatar about the fate of the world battle was coming in her particular life time and only when she's a kid? So they wanted to keep the avatar safe but it's never occured to them "hmm the harmonic convergence will take place when the new avatar is only still 17 we'd better start making some big plans to prepare for it. Like informing the avatar's trainers."
Aang had only a year to prepare for a fate-of-the-world-fight or it was end of the world and he came up on schedule just barely on time, Korra had at least ten years to prepare for a -fate-of-the-world-fight. The fire sages knew this and didn't tell anyone so that Korra can come out on schedule just on time, even though they could have prepared for it when Aang couldn't.
Yeah wan only had a year but did fine, but still if you have time it's wise to use it and take no chances espeacially if its the fate of the world at stake. Also factoring the Vaatu could have been planning and growing in power. Also factoring from the battle sequences:
Korra(with multilife-times powered avatarstate)<Dark spirit.
Wan (With no previous life-time powered avatar state) >Vaatu(ruler of dark spirits)
Seems like a good idea to inform and dedicate more of the avatar-responsible- for-fate-of-the-world time into preparing the for the battle, when time is an advantage many past avatars don't have.