You design something, you build it, by the time all of that is done you are not using the most recent hardware. This is not a problem only for VR headsets, a year is nothing, look at what you get with playstation or xbox.
When you design knowing your limitations, you can build a much more refined experience. A strict focus on optimizations can extract a lot of otherwise wasted power.
Designing custom silicon is an insanely huge job. The cost and time to make the device would've been far higher. Existing SoCs are pretty appropriate for this, tho it is perhaps a shame they couldn't use perhaps a more powerful Tegra or something.