One of the things that slowed it down a lot was the continual development of the Falcon 9.
With the Block 3/4/5 series of the Falcon 9 (5 is the final design and mostly focused on reusability upgrades, 4 is interim) they have a relatively stable base rocket to develop the Heavy variant against.
Yeah, the regular Falcon 9 is so much stronger nowadays, it can basically do what was planned for Heavy back in ~2015. Just at the thrust difference between Merlin engine iterations (https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family) ), and add the fact that the stage is longer than before, and you have a pretty capable base rocket.
With the Block 3/4/5 series of the Falcon 9 (5 is the final design and mostly focused on reusability upgrades, 4 is interim) they have a relatively stable base rocket to develop the Heavy variant against.