I don't know exactly what sparked it. It was either me finding some unique and unused Legendary Items in LOTRO's coding last year or the forums thread wanting some new addition to the LOTRO Store (also from last year), but the developers saw fit to finally introduce these to the game. Almost all of these skins are exclusive to the Store, costing 395 LP apiece. If there's an in-game way of obtaining something here, I'll have made a note below.
Three different sets have been released, the "Soaring Depths," the "Khazad-dûm Vaults," and the "Storied Past" sets. I must point out that these skins have only appeared in the store for limited periods of time, and even then, only one set at a time. Watch the LOTRO Store (or Twitter or the client launcher), as I have, for their next appearance. I'd say once every month or two we've been graced by their presence.
I've collected only certain ones, as I don't have infinite funds to pour into LOTRO's coffers, and I haven't regretted those purchases either!
And, oh yes, I've moved these skins from the other CWP entries to here.
UPDATE: As of 5/9/19, these weapon skins have been made permanently available in the Store!