This sort of thing is the reason why piracy will never truly die. As VALVe's Gabe Newell said, the only way to beat piracy is to provide a better service than the pirates. Unfourtunatly, the practice of M+/Viz passing on series makes it so that they go to the pirate sites instead; International Yuuna fans HAVE to resort to piracy to be all caught up. if Shueisha/Viz really wanted to stop piracy, they really should stop passing on series.
As
@RafGL said, licensing issues. If Viz had the license, they probably would have done so. Hell, they have SnS on their site.
But I'm going to bring up the issue of providing better service. The biggest issue that remains is the distribution. Stores get the issues as early as Thursday. This is because Jump wants all readers to be able to read it around the same time (someone correct me if I'm wrong), even to the remote areas. These remote areas are where early issues arrive, and where Edited Mangas get their issues (before the big sites got taken down).
The easiest answer would be to cut out the early issues entirely and release the issue digitally world-wide as soon as prints are done, or release the chapters before it gets shipped nationwide. Plus, mangakas are usually ahead of the current chapters by 2-3 issues.
Of course, there might remain the issue of leechers still ripping it from official sites, in which the Japanese readers may flock to read. The reason why this could be an issue there would be that Japanese readers may be more fluent in English than we give them credit for. But I seriously doubt this will be an issue since M+ readers will keep rising if they cut out the distribution process, and eventually, leechers will dwindle. Moreover, this hopefully pushes Jump to release their own native digital app.
The final issue that remains is monetization, which they'll have to set a price for their native app. There is also the fact that digital readership is rising in Japan. No exact numbers, but Nakano has confirmed this is the case. And yet, Nakano is
still insistent on returning Jump's print numbers into the Golden Age, and I'll leave it at that.