They would only be defaulting on supported hardware. Nvidia added support last year, but there were still external issues like explicit sync and also Plasma 5 was problematic. Plasma 6 is a giant leap comparatively and only came out in February. There is a lot of Nvidia dGPUs out there. Steam surveys have had it around 70% of steam installs and that includes Intel and AMD apus as well as the dGPUs.