They didn’t cancel it because it was successful. They cancelled it because it was only marginally successful, with big cost overruns, and other technologies proved cheaper and more effective. We don’t have to rely on the speed of lasers to intercept projectiles anymore, because we have better targeting and guidance systems, so we can hit a missile with a much cheaper and more reliable weapon than a laser.
It could only kill missiles in the boost phase, when they’re the most vulnerable. That required the modified 747 to run a very predicatable racetrack pattern along the FEBA or hostile border.
Although, the variable cost of a laser firing (especially a solid-state laser) is signicicantly less than a guided missile. So, you might have a higher fixed cost and overhead, but much smaller variable costs per kill.