Falcon Heavy was specified relatively early in the Falcon 9’s development cycle. They were originally going to do more Falcon Heavy missions but in the meantime, blocks 3 and 4 of Falcon 9 came out and the Merlin kept getting better and better, and with block 5 the Merlin D has almost double the thrust of the Merlins on the early Falcon 9s which is kind of nuts. So many of the missions conceived for Falcon Heavy were ultimately flown on a different rocket- a more capable single stick Falcon 9.