1. Hardware requirements are steep (GTX 970 minimum), so plenty of hardware -- including plenty of hardware that's good for high-end gaming today -- won't be good enough for this.
2. Only Windows support. No compatibility with Linux, SteamOS, or even OSX means that the techie enthusiasts who'd play with it as a tech toy are going to be less interested.
3. But biggest of all: The games aren't there. It's become very clear, both from playing with the DK2, and listening to presentations by Valve and Oculus people, that VR games can't just be regular games with VR bolted on. "Skyrim, but with VR" sounds cool, and makes for a cool five-minute demo, but will just make you sick and be unsatisfying in the long term.
VR games need to be designed for VR in a really fundamental way. It's not clear that there are any interesting games that do this, or that there's a lot of effort going toward making games like this.
If this were a product that were coming out from Nintendo, I'd be confident that they had a good idea about how to adapt their franchises to take advantage of VR in a really cool way, and that there'd be at least a handful of games that made it an absolute must-have. But from Facebook... well, they're a tech company, not a gaming company, so they're depending on someone else to make the games that will justify this thing. Maybe that'll work in the long term, but right now, it doesn't seem like there's any must-have VR game.
Add that all up, and this is a product that won't appeal to most gamers yet, and won't appeal to a lot of tech geeks due to the Windows focus, so... yeah.