Well Vulkan only works easily with Radeon cards. Only certain Nvidia cards cooperates nicely with Vulkan. If you are using a Nvidia that cannot cooperate with Vulkan, then the game will instantly crash. Also Vulkan requires Windows 10, so if you are you like me, with Win 8.1, waiting for Windows 11, then that would immediately mean a lot of people wouldn't be able to play it.
Now if they were making a next-gen game, with UE5, then making it with vulkan would make sense, as any computer needs to be super powerful to run UE5, and thus you'd have a compatible card.