Falcon 9 Replaces Falcon Heavy for Many Missions
When Falcon 9 was first developed, it had a max payload to Low Erth Orbit (LEO) of 9t, and 4.8t to Geostationary Orbit (GEO). Falcon Heavy was built to handle the heavier payloads. However, over time Falcon 9 has been upgraded with densified propellant and more efficient engines, and can now put 16.5 tons in LEO in recovery mode and 23t in expendable mode. A Falcon 9 block 5 in expendable mode can put 8.3t in GEO, and in recovery mode 5.3t.
Without recovery of the center stage, Falcon Heavy can put 16t of mass into Geostationary orbit. With full recovery of all stages, that amount drops to 8t. So for geostationary flights, there is only a small window of payloads where a Falcon Heavy with recoverable core stage makes more sense than a Falcon 9.
So, Falcon Heavy tends to be used for heavy geostationary satellites that require the center core to be expended. Falcon 9 can do almost everything else. Another window might be for a fully reusable Falcon Heavy vs expending a Falcon 9, but those use cases may not be very common.
As for LEO, there is not much market for gigantic satellites in Low Earth Orbit, and Falcon 9 performs so well that Falcon Heavy would be rarely needed. And for heavier satellites, SpaceX is planning to use Starship, and once Starship is fully proven out they want to retire Falcon Heavy and Falcon 9. So further development on Falcon Heavy is probably not in the cards. It's an interim rocket.
Wikipedia -Falcon Heavy
Wikipedia - Falcon 9
Ars Technica - Forget the Falcon Heavy's Payload and Think about Where the Rocket Will Go