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]
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.
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]
[1] https://heatinghelp.com/blog/nyc-one-pipe-steam/