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They used Sinnoh music for Ash's Grand Trial against Hala.
Don't think there's really anything stopping them from using, they just choose not to in most cases.
We don't really know for sure but the older music stopped being used right around the time Kadokawa Corporation acquired Media Factory back in 2011, effectively killing off the Pikachu Records label. Sony Music Entertainment / Overlap handle all the music releases now so the whole thing might be a rights issue where the latter is not allowed access to the former's catalog.
(The older music is still used in variety shows like Pokenchi, however. But live action variety shows in Japan also have an amazingly cavalier attitude when it comes to using whatever the hell music they want, legal red tape be damned)