User blog comment:Akrie/November 27th Events & Updates~!/@comment-26044650-20151127051542/@comment-26044650-20151127055833

The treasures are essentially rotated each season. Some will be removed temporarily. This can be a good thing, because it does increase the chances of obtaining the treasure you want. But it can also be a bad thing because it prevents you from getting the treasure you want since it was removed.

When the next season rolls around, the treasures will be randomly switched (the newer ones will still be added in).

Although it DOES bring up the speculation that Tower of the Frozen Waves may not come to LCR for months since the Treasure Rotation feature was just introduced. :/