Because the land version of Phalanx, the Centurion, is big, needs a large transport and a lot of energy.
https://en.wikipedia.org/wiki/Centurion_C-RAM
Although an auto cannon seems to be the right thing. The current thing seems to be the mobile, smaller Skyranger systems:
https://en.wikipedia.org/wiki/Skyranger_30
That’s still rather big and is a dedicated vehicle. I wonder if there are initiatives of coupling the remote-controlled weapon turrents on normal vehicles with radar and such enabling anti-air capability:
https://en.wikipedia.org/wiki/Remote_controlled_weapon_stati...