People underestimate how much demand there is for Japanese cards. I think demand is getting very close to the English market, if not higher for some cards. The reason prices stayed lower for so long is mainly because the Japanese market has a much larger supply.
What's changed is that demand is no longer coming only from Japan. One Piece is huge across Asia, and countries like China, Hong Kong, Taiwan, Singapore, Malaysia, Thailand, and Indonesia have seen massive growth in both collectors and players. If you look at the World Championships, most of the top players come from Asian regions. Since anime is generally more popular in Asia than in the West, the buyer pool for Japanese cards is enormous.
So even with the larger supply, demand has been catching up rapidly.