In my opinion, the decline in the price of fansite items was influenced by 3 major factors:
1) Fansite Appreciation Day lotteries,
2) removing fansite items from the Market,
3) the introduction of a tier system into the game.
Through the lottery held during Fansite Appreciation Day, the number of fansite items given away during the year increased. However, I think that it is not the amount of items given out that is the reason, but to whom they are given out. They are given to people who did not put any effort into getting them, making the vision of profit make them want to get rid of the new item as soon as possible. They have often undercut these prices in the past to get rid of them as quickly as possible. And this is also where the removal of items from the marketplace comes into play. A person who wins such an item does not have the opportunity to simply sell it through the in-game market, so having to constantly advertise their offer is inclined to let the price down faster. "The item is worth 10k TC? I'll sell it for 7k TC, get rid of it faster!" It only took 2 years to make prices 10 times lower, by thinking this way.
The tier system is not insignificant either. Until now, the determinant of richness in the game was the number of rare items (including fansite items), now people are looking for sources of income to invest in tiering. That's why they are selling off their collections instead of putting in their budget.