It would seem to me that a phased array antenna would be great for spacecrafts since it removes the need to turn the entire spacecraft around in order to point the radio dish towards Earth. Many spacecrafts already have to turn to point their instruments, solar panels and radiation shield in the right direction, so getting rid of antenna pointing should be helpful for both spacecraft and operations design. An interplanetary probe like New Horizons could've been able to send data while making observations and more importantly always been able to receive a last minute command. Station keeping costs could be lowered for orbiting satellites, including the ISS. So why do spacecrafts still carry a fixed dish instead of a phased array?
I note that the 285 kg and 90 Watt Mercury Magnetospheric Orbiter, a planned JAXA Mercury orbiter, will have a phased array for communicating with Earth at 160 Gbit/year, and Mercury is about 90 to 210 million km from Earth. So cost, mass, size, power requirements, data rate and distances don't seem to be obstacles for phased arrays in space. So what is the problem with them? Or are they maybe already taking over?