I don't think many marines are big enough to wield them. Khârn can wield Gorechild because it's relatively small compared to other Primarch weapons like the Lion's sword or Rus' axe, but things like Manus' hammer, Dorn's eviscerator, and what-have-you should just be too damn big for the typical marine to wield. Greatswords after all are typically as large as their wielder. Primarchs are around 12 feet tall on average, give or take. Thus a sword used by them would be at least eleven feet long. A marine is only 7'6" on average.
Any two handed weapon made for a Primarch is going to look incredibly silly next to a marine. Just look at the Emperor's Sword.
It's the same even with a short sword. What's a mere gladius for a Primarch is a lengthy arming sword/longsword for a marine. A small axe like Gorechild is a gigantic one handed chainaxe the size of most full power axes. And so on. This also isn't to say that Marines couldn't use them of course. They could absolutely lift and swing them as they are ludicrously strong in every regard, but there's a point where a weapon is simply too large to be useful in a fight.
Edited by Volt, 01 November 2017 - 04:19 AM.