[techhelpbb]

There is a bit of a catch 22 with the higher on-state resistance.

These robots are being driven by humans.
Human sees robot go too slow...tells robot to go faster.

Higher on-state resistance means that robot spends more time operating closer to 100% duty cycle (not at 100% duty cycle obviously, but more time with the MOSFETs on) than in a situation with lower on-state resistance.
The more time you spend on, the more surges from the motor you dissipate through the MOSFET's junctions instead of the body diodes (never mind the normal currents).