Why are always used N-Channel MOSFET on both High and Low side on BLDC applications?
More efficient, more available and cheaper.

Also usually not any easier or simpler. PMOS requires extra high side gate circuitry anyways above ~20V (limited by max |Vgs| that the gate-source is exposed to when gate pulled to GND to turn it on in a simple PMOS high side gate drive scheme) so you don't save any effort. Max |Vds| is higher so is not the weakest link in a simple high side PMOS scheme. If you need high side gate drivers anyways then you might as well just use NMOS.