If I had to guess, it’s because the price for NIB JJP games is just too high for a lot of people. There aren’t as many people able to buy games at that price point. So when it comes time to sell, there aren’t as many buyers in the secondary market either.
There’s also the feeling that you need a decent deal to buy used vs new. And I believe that amount changes based on what you can afford.
For people that have $5k to spend on a game, maybe $500 in their pocket is more beneficial to their financial situation than $1000 is to someone in the market for a $10k game.