>>55371
I can't believe I'm about to go into lore and headcanons fur doom of all things but whatever.
Doom is interplanetary, not interstellar. In general, the tech level will be lower than like halo or something. We can tell from the zombified soldiers that the standard issue is still ballistic weaponry, so the plasma gun and bfg are likely special/experimental. In addition, the large number of zombies act as a reliable stream of ammunition fur the weapons they use.
Of the weapons with readily available ammunition, the double-barrel makes sense as the one you would carry around when you're not sure what to expect. A lot of instantaneous power without the risk of blowing yourself up like with the rocket launcher.