We plan to support s&box over multiple decades, so we want to launch on a modern graphics API and have no baggage when it comes to supporting new GPU features.
We don't want to end up in a position where we have 4 different render pipelines.
Source 2 already has Vulkan support. Valve sponsor and contribute to Vulkan.
And they've been willing and kind enough to keep sharing improvements to their Vulkan render system with us.