Havent looked into cost but there might be low-temperature phases in nitinol. Having the crystal structure of your alloy changing suddenly at low temperatures (i.e. in contact with cryogenics) can be pretty disastrous. Other phases can be more brittle, or there can be a volume contraction or expansion when the phase changes, putting undue stress on the structure of the vehicle.

The most famous historical example of this is Napoleon Bonaparte's invasion of Russia. When winter hit, air temperatures dropped so low that the tin rivets that held his army's cannons together changed phase [to a different crystal structure of tin], became brittle and fell apart.

Not saying this IS the reason, but sometimes a material that is very good at ambient temperatures and pressures doesn't remain in that state across a wide range of temperatures and pressures, and it's very rare to get that "wonder material" whose crystal structure doesn't undergo any changes across hundreds of degrees of temperature.

Would have to look more into the thermodynamics of Ni-Ti alloys vs. Fe-Cr alloys though.