Just checked both new and Old Winner Sites Notes down below which mentioned this.

Old Winner Page
If Square Enix determines that a particular entry is especially excellent, Square Enix may decide to implement the design into the game. Such implementation is not guaranteed and is not a part of the contest prize.
New Winner Page
If an entry is deemed to be excellent, Square Enix may decide to implement it into the game in the future. Note that there is no guarantee that winning entries will be implemented into the game.

So it seems that they might not add some of the previous Winners at all?
That's sad cause some of the Older Ones are still pretty to have.