Closed loop control of brushless motors is just more complex, in addition to needing 3 phase AC output, you also need either hall sensors or an encoder of some

Closed loop control of brushless motors is just more complex, in addition to needing 3 phase AC output, you also need either hall sensors or an encoder of some kind to be able to start the motor smoothly, and you need a dedicated IC or MCU for each motor to manage commutation and read the sensors. I don't think FOC type controllers were anywhere near common back then either, which is needed to run a brushless motor smoothly. There is just so much more that can go wrong with a brushless setup, vs brushed where you just apply power and that's it.