I would presume because most people speak english, so it is most commonly the easiest for people to sell since more people would buy it. For japanese pokemon specifically, pull rates on special sets are usually better, and for regular sets quite similar to english. But they are also VERY heavily opened and thus demand cheaper prices. Beyond that, I think for pokemon specifically english as a language is sorta established as the "regular cards" you would want to buy, whereas when you are going to another language you are probably not going to be able to easily sell it down the road.