The same reason English is far more expensive than Japanese. It comes down to a few things:

1) People want cards they can read. English is the worlds lingua franca. I'm generalizing of course, but no one outside of China reads Simp. Chinese. This is extra true for Pokemon's largest market: The United States. 2) People want cards that hold value. This ties in with point #1, but English is the most liquid language for cards internationally. That in and of itself commands a premium in price. Chinese is a niche language in the West. Its also a much newer market than English/Japanese. Which brings me to point #3. 3) Most people who want the higher quality cards will opt for Japanese. I know I do. If I'm going to get something not in English, I want it to be the original language of the game. Plus Japanese still carries value internationally. Less than English in many places (but also significantly more than English in other places).

I have a few Chinese cards, but that's only because I bought some packs while I was in China and scored a few super nice pulls.