Volkite were a new technology when the Horus Heresy broke out. Due to the schism on Mars and the Mechanicum, it's not unreasonable to see how that technology could be lost.
Gravity Weapons work very differently in 30K then
40K. A possible fluff reason would be that the gravity weapons of 30K are not the same weapons as the ones in
40K, and that gravity weapons in
40K are a "new" style of tech.
Space Marines are not lacking for weapons or tech in
40K, and they really don't need another buff. They would be the only armies to benefit from bringing 30K weapons into
40K.
But hey, Forge the Narrative and say your Chapter of Space Marines liberated a Forge World with all these wonderful, old weapons.
Automatically Appended Next Post: beast_gts wrote:Volkite was being phased out at the start of the
HH. 30k graviton weapons are different tech to
40k grav.
The fluff explanation is that the forgeworlds &
STCs for these things were lost during the
HH.
Horus Heresy Book One: Betrayal says Volkite was a relatively new technology, but was very difficult to produce and maintain. Bolters were much cheaper, easier to make and maintain and that's why Volkite was being replaced.