Ground source heat pumps install cost is very high when used in cold areas because of the frost depth caused excavation costs; significantly higher than the cost of the heat pump unit and parts themselves. It can be good if that disproportional initial cost is paid. But it's not the same class of purchase. This cost is reflected in the lack of demand and so lack of (solvent) installers.

If the big initial cost is rolled into home construction it would be less daunting and more normal. So I expect newer homes to adopt heat pumps in cold areas but with little adoption in existing homes.