There may be legal issues, Pierre-Loup at Valve (who incidentally recently tweeted about NVK's progress) used to work at Nvidia on Linux driver development. There's a good chance a lot of Valve's employees have signed NDAs too. They've been leaning heavily on partnerships and contract work (like with Collabora, who also host NVK).

Valve want to extricate themselves from Windows, their incentive is to break free from "Wintel". Steam Deck is a priority, but SteamOS is clearly built to extend beyond that and in order to have any success on standard PCs, Nvidia GPUs need to be supported well.