They don't usually release the soundtracks for the third versions. The only time they've ever done this was for Black and White 2, which included much more new music than normal, and they still had to pad the soundtrack out by including the added tracks from Emerald and Platinum as well. Ultra Sun and Moon simply doesn't have enough new music to justify its own soundtrack release.
Not to mention, it should be relatively obvious at this point that there is no such plans, as it's been two and a half years since the games released, and soundtracks are usually released within six months or so. Sword and Shield doesn't have its soundtrack released yet, but it's beginning to look like that might be in order to include music from the DLC.