It's mostly due to the competition from the "normal" Falcon 9, which is quite capable and could be used for most of the existing payloads.

Besides that, currently the payload is limited by the size of the fairing. SpaceX is working on the bigger fairing, but it is not available yet.

falcon heavy was always this awkward middle child relative to F9 and Starship. Like iirc it was announced pretty close to F9 blk5 going to production, but very late, and then the starship concept took off and consumed resources and attention. The moment starship is operational then FH is cancelled I bet. I bet F9 hangs around even after Starship is operational however.

There were no missions... there are now 4 or 5 more on the manifest for the next 12 months.