Japan has this strict copyright laws. Just recently, furransu, one of the old-time provider of J-drama raws, decided to stop because of the "multiple arrests" in Japan. In his/her word, "they are really cracking down on online piracy". So, it's more difficult to find J-drama sources.
As the others said, they also don't prioritize marketing internationally, so there aren't many people aware that such and such drama exists.
Moreover, there are less fansubbers for J-dramas as compared to K-dramas. Most of them take 1 drama per season and it's depending, of course, on their interests, so a lot of the dramas don't get English subs.