TL;DR - it is more practical and cost effective to fit a Truman-class ships with 42 x 40mm PDCs vs one or two gigawatt lasers.
In The Expanse, ships travel at hundreds of kilometers per second and torpedoes can travel even faster. If you account for relative velocities between a ship and missing, then the apparent velocity of a torpedo may actually only be a few dozen kilometers per second (from the point of view of the targeted ship). That means a torpedo can traverse a distance of around 300 - 500km in a mere 10 seconds.
Lasers travel at the speed of light but beam quality and diffraction drastically decrease their effective range since the beam will spread out as it travels. Range can be increased by building a bigger aperture (structural/material stress and mount size become an issue) or by increasing the power output (waste heat, thermal stress, melting of components, and power generation become an issue), but you are still looking at a fairly large weapon with an effective range of at best a few hundred kilometers and output of a few hundred megawatts (which would require one or more gigawatts of input power to achieve). Also, the greater the range to a target and/or the more armored a target is, the longer a beam needs to remain on it before sufficient damage is done. This task is made even more difficult by the high velocity of the target and the constant adjusting of the laser to focus on the same spot.
So if you have a small swarm of missiles (say six) that are traveling at 50km/s relative to a target defended by a giant laser that maxes out at say 300km and requires one or more seconds to kill a torpedo and acquire a new target, at least two will get through the defense.
Now let's look at PDCs. While they may have muzzle velocities of only a few kilometers per second, they have several things going for them. The lack of horizon in space means that torpedoes can be tracked the second they are launched. At the extreme velocities involved, torpedoes generally cannot afford to maneuver once they are in the terminal phase of their attack or they risk flying past the target completely. So the closer a torpedo gets to its target, the more "straight and steady" its course becomes. A PDC can track the torpedo and once the torpedo enters its terminal phase, the PDC can lead the target and begin throwing out a stream of shells and expect a hit.
Think back to battle between Donnager and stealth ships. One of the crewman says something like "I've never seen torpedo guidance systems this good before sir, they are pushing our PDCs to to the max." He likely meant that those torpedoes were still able to maneuver to some degree during their terminal approach, which made it hard for the PDCs to predict their course.