Umbra is rare, just as rare as artifacts for Daedra's Champion such as Mace of Molag Bal. To keep lore intact, there could be only one of each, like Emperor. Ri'Dariit likes idea, yet not fair for other players not having it. Player will complain, Makers have enough problems already.
Fusozay Var VarJaji kor nirni. Ri'Dariit ahnurr shir Gouranga an vasa rid-t'har.
@xy_productions , he didn't necessarily say he wanted it to be available to players, he said he wanted it to be featured. @annarr1117nub18_ESO it's fairly obvious Umbra (the sword) would never be available to players; However, I wouldn't be surprised if later on down the pipe, in future expansions, a Clavicus Vile storyline is introduced that gives us some more insight to Umbra, not just the sword, but the spirit trapped inside of it. The Umbra isn't too much different than Warcraft 3's Frostmourne, a soul trapped within consuming the souls of its owners. Keep in mind however, we never really know when Naenra Waerr creates it, so we don't know if she's created it already (since we come before Morrowind, Oblivion and Skyrim) But we do know by the time of Oblivion, Clavicus Vile Receives the Umbra Sword (Hence why it makes no appearance in Skyrim.)
For all we know the sword may still be hidden, or traveling with some unlucky sellsword, having his soul defiled by it this very moment...