If there were only a single major vendor, then they would have an effective monopoly which would take years to catch up to, such as iPhones/Android. Instead, we had both Vive and Oculus release at around the same time, and both have seen reasonable success. Prospective buyers worry that they'll get locked into one ecosystem, hoping that the decent games on other platforms get ported over eventually. That indecision probably also stops a large number of prospective buyers from purchasing either platform, because they would rather play it safe than to spend $800 on backing the wrong horse. I know a guy that got burned buying an HD-DVD player, it sucks.
With a shared standard, games developers can build to a common target, and consumers can buy hardware based on actual hardware differentiators. Oculus will be able to say "buy our headset, it has better feature X and it supports all the Vive/GearVR/Cardboard games!" Customers will feel more comfortable just picking a horse, knowing that even if it doesn't win it'll at least finish the race.
With a shared standard, games developers can build to a common target, and consumers can buy hardware based on actual hardware differentiators. Oculus will be able to say "buy our headset, it has better feature X and it supports all the Vive/GearVR/Cardboard games!" Customers will feel more comfortable just picking a horse, knowing that even if it doesn't win it'll at least finish the race.