This is my understanding, not 100% sure it's all correct...

It's not mandated for US 121, and never will be for retrofit. It might possibly become standard or even mandated for future new aircraft.

It's a additional cost, complexity, and maintenance. Also will need regular database updates since you don't want to fly down with a synthetic image of the runway environment in summer and then have to transition to a view of real world winter for example.

As you say it may have more utility for bizjets that flt into smaller, less-equipped airports. Some US airlines were early adopters of HUD technology since they fly into smaller, remote, less-equipped fields... those might go for synthetic vision.

Also training... depending on how good it is, it might require significant additional pilot training to use in low-vis, and that would probably include on-going recurrent training (more cost). 121 is pretty good with CAT 11/CAT III autoland approaches, we're already trained and comfortable doing that and don't need to see the runway before we arrive. There do not appear to be any safety issues with CAT II/III for the most part. Again, it could be helpful at smaller airports.

So it will probably come eventually but there's no rush to spend the money on retrofits for most 121 operators.