The CO2 resonator requires much more energy and is larger. Electricity is used to combine gases to create the laser beam. Mirrors aid in the light's intensity and prepare it for exiting the resonator. The beam must travel along a path that includes several cooled mirrors to reach the lens after it has exited the resonator. This causes the beam to lose power and quality.
Another consideration is the fiber laser’s briefcase-sized modules—they allow for redundancy. If one module has a problem, the resonator doesn’t shut down completely. The fiber laser is redundant in a way that the other modules can produce more power temporarily to support the down module until repairs can be completed—which, by the way, can be done in the field. Other times the fiber resonator can continue producing reduced power until repairs can be made. Unfortunately, if a CO2 resonator has an issue, the entire resonator is down, not just in a reduced-power mode.