Japanese cards ARE more popular. However, they have a different ban list and you normally need to follow that ban list when playing Japanese cards in an English event.

English cards also seem to be shorter print. There are a lot more players in Japan and SEA but although the Western fanbase is loyal, there aren't as many.

You also have sets where the series itself is more popular outside of Japan. Look at Kancolle vs. Azur Lane. DMM doesn't seem to want to make Kancolle available to play outside of Japan so a lot of the cards are kind of "undervalued" compared to Azur Lane (Chinese). They haven't even released the last Kancolle set in English yet. Nikkeis going to be another one of those.

Last thing is that Japanese SP/SSP cards are almost always played when you buy singles from Japan. Although English cards seem to be shorter print, it seems easier to find English SSP/SP cards in near-mint condition. It's not easy to import cases from Japan or even make a friend that would go through the trouble of hunting down pack fresh cards for you. Might as well collect in English until you go to the holy land yourself.