Vulkan was called Mantle before AMD gave it to the open standards group. Mantle existed before Metal and Apple knew it was being made into an open standard befo

Vulkan was called Mantle before AMD gave it to the open standards group. Mantle existed before Metal and Apple knew it was being made into an open standard before they released Metal. AMD and Khronos were open about their plans and Apple was/is part of the group. To answer your question, Khronos are trying to promote an open free API to help developers save time and effort, picking Metal as a closed source and late arrival would be unworkable.