There are many reasons actually
For one, since it releases relatively late in the cycle. About half way through the cycle actually (If we are to believe that this cycle will be shorter than the 360s which seems to be what many publications are pushing).
Second, VR is still very much a niche market for the foreseeable future. Even though I think this will change, it will take a whole lot of word of mouth to convince casual players they have to shell out a few hundred bucks and cut off the world around them.
Additionally VR is very family unfriendly. No sane parent would buy their child a VR Headset so he could shut himself out of human interaction. A serious image change for VR has to happen first, which is still far off. Rather than getting families together (like the wii or kinect) it rather looks like it pushes them apart.
So you are stuck with tech and gaming enthusiasts who have enough money to afford a ps4 and a morpheus. And I think most of these won't care all that much if it's 250 or 350. If you can get 10 million with 250 and zero profit or 7 million with 350 and 700 million profit they'll take the 350.
If It's not 149 with a game casuals won't stumble over themselves to get one.
So if they get to 10 million units sold within the barely 2 years before the next iteration is announced, I will be seriously baffled, and I think with that install base they wouldn't put too many big titles out for it, but rather push them to morpheus 2/ps5
Further, they can push this as a platform as much as they want, it will always be a peripheral in the public eye.
All in all I think that explains quite well why I think this is to check/prepare the market for ps5/morpheus bundle. However, I could be wrong and the HMD will be 199, everyone will buy one and VR will be commonplace in every household within two years. I seriously doubt it though.