Wikipedia says NRO launches are $440m, so the price difference is really really big, which is the reason why this is the last launch - not even the US Government has that kinda cash for launches.

The other reason is there's a lag of several years between when a capability is established (Falcon Heavy) and when large customers can take advantage of it. For example, we really shouldn't expect Starship to make a dent in way satellites and telescopes are designed and built until the 2030s.