Modern RPT jets can be RNP APP approved easily, although at a cost to the individual airline. GC airport investing in an ILS will cost them.
So you've done a cost-benefit analysis on say a half a dozen airlines individually getting RNP-AR approvals (and maintaining their currency) verses plopping in a ILS, not to mention the smaller operators that don't have a hope of ever attaining RNP-AR capability?
After the recent shenanigans, it's pretty obvious the evidence is now overwhelming for some sort of precision approach, and GC certainly wouldn't be putting in an ILS gratis. They'd up the landing fees. But why's that different with any other piece of infrastructure? Service providers the economy-over upgrade equipment/services and then charge for it.