Largely, the matter of size, total volume of refrigerant.

Liquid is incompressible so you can't really run Carnot cycle on liquid alone. Gas alone, due to low density, has very lousy specific heat - can't transfer much heat per unit of volume. So, for refrigeration on gas alone you'd need lots and lots of gas volume to circulate quite fast, making your freezer unreasonably big.

By only keeping a relatively small part of the circuit filled with gas, you assure a decent amount of slowly moving, efficient heat-absorbing coolant (cold liquid evaporating in the evaporator) and heat-expelling coolant (gas compressed into liquid cooling down in the radiator), while only a small part of the circuit carries large amounts of gas, fast, between the evaporator, through compressor, into the radiator, providing the pressure transition required by the Carnot cycle.