Because it wildly under-performed. I've tried it; amazingly disappointing.
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...and I was so disappointed that Apple has nothing of the kind.
Cardboard works surprisingly well. Pop in the intended phone, get a good-performance virtual world. $20 (you've already got the phone, so that's already a sunk cost). Sure, Oculus is far superior - but Cardboard is accessible. Set some basic standards, drop hardware entry cost to near zero, and start drawing people into the new VR wing of the walled garden. Make it so easy it's hard not to. A few hundred dollars will make newcomers balk, but at the price of a couple lunches you can get people in - if it works.