I'm sure there are a number of contributing factors as to why, but based on what I've researched and observed, I think the main issues are with accessibility and marketing. One of the reasons why Kpop has risen in popularity over the years is because it knows how marketable it can be internationally, especially towards Western audiences, and adapts accordingly. Whereas with Jpop, unless you're deep in the community, it's not as easily accessible due to there being so many distribution restrictions. This has been made painfully clear when Avex recently announced a ban on exports of their stuff, making it even harder for international fans to buy anything directly. You can't find much of anything on YouTube anymore, which is how I discovered most of Jpop back in the day, and if you do it's only "short versions" of music videos or songs.