SM focused a lot on Japan's music market with artists like BoA, SNSD and TVXQ, they all had huge success there, many albums that also sold million copies.
But after them, SM started focusing less and less on Japan, their artists are lucky to just have 2 albums or even 1, mostly of their songs are just japanese versions of korean songs with no new music material to exclusively promote in Japan.
Super Junior just had 3 japanese albums in their whole career and a few singles
EXO just one studio album and 2 single albums, after Countdown SM literally gave up with them in Japan
F(x) just had 2 singles
NCT 127 only had one studio album and 2 mini albums
Red Velvet one studio album that is coming this year, and 2 mini albums
Why's that? Other companies are still focusing on Japan's market with their groups and having japanese fans in a fandom can contribute a lot to album sales.
What's stopping SM from doing the same when Japan is the second largest music market in the world?