I don't think it's hard to understand. Apple wanted an easy to use API that could be extended and updated easily. Vulkan is an extremely complex API that foregoes all and any developer ergonomy to facilitate quick driver development on as many hardware targets as possible. Consequently, Vulkan's design choices are driven by the least common denominator. The goals are just too different.
There is at least some evidence that Apple was interested in supporting and shaping Vulkan (they were member of the initial working group), but I suspect that it very quickly became clear that the committee is going into a direction they were not interested in, so they noted out.
Still, I don't think it's correct that Apple is opposing any kind of standardisation in this domain, they just want something more aligned with their vision. They have been very active with WebGPU, which is shaping up to be a very nice API, and it has inherited a lot of good design decisions from Metal.