Probably because people will steal the heat pumps.
There are apartment buildings going up in NYC with one-pipe steam heat, steel pipes, and cast-iron radiators, because there is nothing outside the boiler room worth stealing.[1]
Nothing to steal here, except for the pump itself its all extremely deep underground.
Good luck stealing a district-heating ground loop. What would the average even do with a water/air or water/water heat pump sized for 800+ units?
The heat pumps are located where the boiler room would be ;-) so... they're about as easy to steal as a boiler. Size is also similar - not that easy to stuff into your pants and walk of whistling.