It's uncomfortable, isolating, constraining, expensive, anti-social and its use cases are at best temporarily impressive but quickly grow old.
The entire product category simply sucks. I'm sure the incidental 3D movie experience is great, but most people don't watch movies every single day for hours on end whilst sitting alone on the couch.
It's slow to operate, can't easily be shared with others and having gigantic screens surrounding you is a useless novelty without utility. Watch people with big monitor setups. They watch the middle one whilst the other 2 are stale and only get the occasional peek. This super-multitasking idea is a myth and not how people work.
Consider how when people own both an iPhone and an iPad, they almost always use the iPhone, even for use cases in which the iPad is superior. It's quicker, more flexible, hassle-free, always ready.
Another issue is that the type of content/experience that would truly shine on such a headset, is impossibly hard/expensive to make. So that leaves out 99% of app developers.