Because of availability.
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I didn't think it needed an explanation util I talked with a collegue of mine who almost completely disagreed. Here are four reasons why I think that, unless corporate shenanigans fuck things up, a descendant of Google Cardboard will still exist in 5 years, while currently hyped darlings like Oculus Rift, Samsung Gear and others will be forgotten.
#1: VR can only succeed if it's massively available
If there is a future in VR, and to me this question still remains unsolved, as there doesn't really appear to be a "killer application" for it (much as there isn't one for smart watches), it will need to be massively available. By this, I mean readily affordable in markets such as India and China.
That said, the same as there is currently a market for hugely powerful 3D graphics cards and pimped-up desktop computers, there will also be a market for fancy and expensive VR rigs.
#2: Everyone who wants one, already has a smart phone
Given a choice, why would anyone buy another piece of expensive equipment if they already have one which is already highly sophisticated and reasonably (depending on its price) powerful?
A computer ready to run Oculus VR costs about $1000, in addition to the $600 or so set itself. This is, currently, absolutely an enthusiasts-only territory budget-wise.
A nice mid-range Google Cardboard clone which won't fall apart in your hands and is soft on your face costs about $30.
#3: Cardboard requires no standardisation
There is nothing to standardise, as evidenced by Cardboard being supported by both Android and iOS.
Of course, some finishing touches are actually needed for Cardboard to become more immersive, and it will probably not end up being a passive device (though - who knows - it may become powered through the phone itself!).
At this point, I'd say the #1 thing which needs to be added to Cardboard is some kind of a controller, whether a touch-pad, a joystick, or some fancy Kinect-like tracking, or even a futuristic interface with smart-watches - anything will be better than the single button which exists right now.
Next, some kind of sound gear, like speakers within the hud, will be nice to have, to support directional audio.
Note that both can be connected to any phone by an existing technology like Bluetooth.
There need not be a new standards war. As history teaches us, the money isn't in the hardware, it's in the content, so the less time companies spend delusional that their particular proprietary version of hardware will win the future VHS-vs-Beta foolery, the better.
Also, the future is not in perfect technology, but one which is simply good enough. People will settle for less dense video resolution, and a moderate amount of lag, if they have that killer-app feeling.
#4: Availability means more people innovating
Out-of-the box new innovations will (as history proves repeatedly) not come from existing large companies, even if those are Google. I predict a killer-app VR innovation will come from an enterprenuer in some backwater village no-one has ever heard of, because he was able to afford a set, and because he already had a smart-phone.
Any expensive (or closed) platforms will naturally fail and, at best, become a footnote in history.
The future might bring a convergence between currently high-end high-priced devices and smart-phones - I consider that to also be a valid scenario, if the result is massively available and as affordable as today's low-end smartphones.
An alternative solution would be that technology becomes so cheap that a "stand-alone" VR set + a smartphone becomes as affordable as just a smartphone is today. Which would also be very nice as it would mean radical innovations in a lot of tech sectors. I won't be holding my breath for this scenario in the next 5-10 years, though.