Without going into detail; it's because only AMD & NVIDIA cards support bindless textures in their hardware, there's one major Desktop vendor that doesn't support it even though it's DX11 HW. Also take in mind both Vulkan & DX12 want to support mobile hardware as well.
There is something I don't really understand in Vulkan/DX12, it's the "descriptor" object. Apparently it acts as a gpu readable data chunk that hold texture pointer/size/layout and sampler info, but I don't understand the descriptor set/pool concept work, this sounds a lot like array of bindless texture handle to me.
You will have to give the API a table of textures based on frequency of updates: One blob of textures for those that change per material, one blob of textures for those that rarely change (e.g. environment maps), and another blob of textures that don't change (e.g. shadow maps).
It's very analogous to how we have been doing constant buffers with shaders (provide different buffers based on frequency of update).
And you put those blobs into a bigger blob and tell the API "I want to render with this big blob which is a collection of blobs of textures"; so the API can translate this very well to all sorts of hardware (mobile, Intel on desktop, and bindless like AMD's and NVIDIA's).
If all hardware were bindless, this set/pool wouldn't be needed because you could change one texture anywhere with minimal GPU overhead like you do in OpenGL4 with bindless texture extensions.
Nonetheless this descriptor pool set is also useful for non-texture stuff, (e.g. anything that requires binding, like constant buffers). It is quite generic.