As title, I always see an N-Channel MOSFET on both HIGH and LOW side of a 3-phase BLDC motor driver.
i.e.
Can you explain me why is better to use 6 n-channel instead of 3 n-channel and 3 p-channel?
p.s. In this case there isn't high side driver to PWM mosfets.
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.
PMOS is more expensive and tends to have more gate capacitance for the same RDS_on (P-channel semiconductor doesn't conduct as well, so everything is bigger).
Use of N-channel FETs isn't exclusive. The gate drive is simpler at low voltages (below 15V or so) if you use PMOS on the top side; if your current needs are also modest it may be cost effective to use the more expensive pass transistors so that you can use simpler gate drive circuitry.
But definitely as soon as the supply voltage starts getting close to the maximum gate-source voltage, or if the FET size and cost is exceeding the size and cost of the gate drivers, you should just use NMOS.
