Well...
Don't underestimate lasguns. And Ork /can/ take a full barrage before dying, but one shot from a laspistol could also fell and Ork. A space marine's armor will protect them from lasfire, which is why guardsmen are instructed to aim for joints when fighting the traitor legions, because a lasgun is wholly capable of defeating them at that point. A reasonably talented guardsmen is capable of defeating any foe with a lasgun (provided they survive long enough). This is not to say that other weapons do not have their bonuses.
Plasmas, meltas, etc, are really useful, but in a mass battlefield they can be as dangerous to your own troops as the enemy. They have a tendency to explode when taking damage, they overheat, they require rather large firing lanes so they don't burn the face off of your friend, and most have terribly slow rate of fire and low accuracy at the best of times.
Lasguns are the all-around best weapon a guardsman can wield and expect to have optimum efficiency when confronted with every situation.