Great question! I'm not sure of a good answer though, maybe try emailing the authors to ask?

Rocinante had a separate bank of batteries/capacitors for its railgun. That had to add a fair amount of weight, which reduces maneuverability. Sure, the drives they use are hand waving magic future tech, but a ship the size of the Rocinante might not be able to power 6 lasers.

There's also the durability of the PDCs to consider: if the heavy duty power cabling is damaged, lasers can't fire, but a PDC just needs minimal gimbal power and a control circuit. PDLs need a lot more stuff that might not be very tough, like capacitor banks.

Plus PDCs can load different ammunition. Flak for missiles, armour piercing for ships.

Or it could just be an oversight by the authors! I'm not sure if any of my reasons are valid, it could be anything really.

PDCs have transfer of momentum going for them. Hit something, not only do you break it up but unless it's hot exactly head on you've transferred some of the momentum in a sideways direction. Get it far out enough, and the trajectory change is enough it misses you. Lasers would not have that going for them, save for the small amount of ablation they cause on the target.