Apple doesn't have infinite software engineering resources. In fact, given how slowly they adopt new OpenGL stuff, it's probably safe to say that they're pretty constrained on software engineering resources when it comes to low-level graphics technologies. So it's not surprising at all that they've chosen to focus on Metal instead of implementing Vulcan.
As for why they did Metal, they most certainly started work on it before the Vulcan project started. Wikipedia says[1] that the Vulcan project was started in July of 2014 (and then announced publicly at SIGGRAPH, which appears to have been in August). But Apple released metal with iOS 8 in September of 2014, and they'd obviously been working on it for more than a couple of months.
In an ideal world, Apple would support both. But it's not an ideal world and supporting Vulcan would take resources away from their work on Metal, and that's not something that makes sense for Apple to do.
> So it's not surprising at all that they've chosen to focus on Metal instead of implementing Vulcan.
That's not the real reason, unless Apple will claim that they can't plan things right. They knew Vulkan was coming, so they could allocate their resources to it from the beginning. But they decided to push for Metal instead.
TL;DR: Vulkan wasn't born out of nothing. Things started with Mantle. Apple didn't want to share effort in both directions joining the open API (which ended up in Vulkan), and made their Metal for themselves only, using the same Mantle as the source.
Their hardware vendors already wrote drivers that support vulkan for other platforms. If Apple is so resource constrained (despite sitting on billions of dollars in cash) why do they insist on building it themselves when they can't keep up?
Sure, I'm not saying they are looking for excuses. They are just lock-in jerks and never really hid the fact.