Perhaps it’s simpler and more efficient speed control circuitry. For a DC motor you just need to step down the DC voltage from the batteries. For a brushless motor you basically need a variable frequency inverter that’s necessarily going to be more complex and possibly less efficient.

Brushed motors are readily available in the appropriate sizes in small quantities and used. Brushless motors not so much yet. Then you also have to use the OEM inverter which will take some reverse engineering or bring your own inverter. People are doing this with car conversions, but it's early days and anyone can wire up a brushed motor.