It's cross-platform, safe, (reasonably) standardized, and fast (in roughly that order of priority). Even if you only care about the last three, there are not all that many competitors outside of maybe Metal, which is not the API what most people are thinking of when they say they don't care about working cross platform :P Vulkan emulation exists for some older machines, but it's generally slow and janky due to the impedence mismatch of trying to build a very low level API on top of a high level one.