While lasguns themselves are a relatively new concept, lasers have been around for quite some time. Since the 1960s, lasers have existed in various forms, though all were much too weak to deal much damage to human targets.
During the age of the Imperium, Lockheed-Martin was more or less coerced into mass-producing laser artillery for the Imperium's military. This often ended poorly, as the lasers had a tendency to jam up when firing over fog and smoke, a property which was heavily exploited by the Red Army. Increase the beam's power, and the ionization of the air surrounding the beam would not only destroy the lasgun but also injure allied troops, perhaps more so than their targets. As such, all the Imperium lasguns remained mostly as museum specimens or simply left to rust (although fibersteel can't really rust, but that's besides the point) in warehouses.
Lasguns found some more use during space exploration before the birth of no-ships, where early exploration ships used them to destroy small asteroids, or simply to knock them off course. Laser satellites were also deployed to clear out fields of space trash, though this was not a very widespread use. With the advent of no-ships, lasguns became practically useless again. Due to warfare in space, well, never happening, ship-mounted lasguns as weapons were never very widespread.
It wasn't until some years later that a new way to use lasguns as a personal weapon was found. By having a single gun project many beams to converge on a single point, the air could be turned to plasma (or the target burned) far from the gun, allowing for medium to heavy long-range beams without any harm to the user. This still had some drawbacks, namely that enough vapor or smog would still stop it. However, since there was no risk of a stray projectile piercing the walls of the vessel, they found much use among crews of no-ships.
Although again, nobody really ever used them because there was nobody to fight.
The ghola Haika used one such lasgun while being forced to participate in the Second Killing Game aboard the Krasnyi Fujisaki.
On a final note, several large lasgun platforms are currently under construction to defend earth under the off-chance that any no-ships should encounter hostile foreign life, although this has been deemed very unlikely for many reasons by the world's top mentats.