There's a reason for that:
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As much as I'd like more MoM I'm really glad for this decision. DLC that's released at the same time or shortly after a game's release means that the content was developed at the same time as the base game and could have been part of it instead of being sold separately. And the DLC for Theatrhythm was 99 cents a track. Thanks to Nomura's intervention all the tracks that could have been DLC are in the base game instead.
MoM's success might have changed this, but if they did the DLC takes time to make. I'd say that the most likely scenario would be a bunch of KH3 tracks released around KH's 20th anniversary. But don't count on it, they could also just want to move on to their next project.