There have been 11 Falcon Heavy launches (as I said in my comment) in the almost 6 years since the first flight (February 2018), roughly 2 per year. There were almost 150 Falcon 9 launches in 2024 alone.
What else would you call this than "not a lot of demand"?
Falcon Heavy has done far too few launches to accurately predict the demand.
They may be supply constrained as well. Only one of the 3 Falcon launch pads can support Falcon Heavy and it must be reconfigured to go between regular and Falcon launches so each Heavy launch restricts Falcon launches for a week or two on either side. The second launch site at Vandenberg is being built right now and will support both regular and Heavy launches as well. For every other launch company reserving a launch site for a month for one launch wouldn't be a problem but for SpaceX that will prevent scheduling of 4-6 other Falcon launches.
SpaceX might be pricing Heavy launches high enough to dampen demand until they can support more launches.
Because there is not enough demand. If there would be more demand there would be more launches. Because why not?
(Are you sure that you are talking about Falcon Heavy? The heavy launcher which is basically 3 Falcon 9 boosters bungee corded together[1]. First launched in 2018. And not Starship which first launched successfully in 2024?)
1: Not really. Just a joke. Before someone nitpicks
Why are you conflating demand with launch cadence?
Falcon Heavy is in R&D mode, which is why there have been fewer launches. That has no bearing on the demand for it.