Testing/Validation and bugfixing. Just having Vulkan isn't enough to enable it by default, everything actually has to work right. Even for Android this is only

Testing/Validation and bugfixing. Just having Vulkan isn't enough to enable it by default, everything actually has to work right. Even for Android this is only for specific types of devices. You should be able to force enable it on Linux right now though. It's just not GA quality guaranteed.