That's a big apples to oranges comparison, but breaking it down anyways... for an apple watch all you need is a smartphone which the majority of people have. For one of these VR HMDs you need a 300 dollar graphics card and a capable computer, which is a much smaller market segment for a device which has only been out a handful of months.

So, how will this change? Well GPU's are getting increasingly powerful, Nvidia will likely debut a GPU that is powerful enough for a Rift/Vive on a laptop this year, and AMD will release a graphics card powerful enough for VR on desktop at a price point less than 200 bucks.