A bit of gross generalisation, but it passes the 'sniff test' if you will:
VR is looking like big business to a lot of investors.
And Big Business is all about risk. When you're talking billion-dollar acquisitions (or two!) of upcoming VR companies with prototype hardware, risk is all about minimising the damage should your investment go off the rails.
These little issues are fine for investors, but on the inside, they'll want a return on their investments soon enough. Billions of dollars in the tech world can evaporate alarmingly quickly, so risk is correspondingly high. This means the business terms are tight - returns will need to be high (and met on definite, pre-determined dates and insurances can be crippling.
Making both platforms (and others) easily support each others software is adding additional risk; its something investors in one company cannot account for - the actions in another company (HTC/Valve). Locking down software for exclusive use 'compartmentalises' the risk, quantifying it based on the software sales projections (meaning hardware sales are more likely).
Remember in 1999, when we saw the first screenshots of Bungie's Halo on the PC, and wowed at the vehicles, detailed Dyson-esque landscapes? Then Microsoft bought Bungie, and used Halo as the 'killer app' to sell the X-Box. Halo on the PC evaporated overnight, X-Box sales were great (in part due to Halo and other titles). Software exclusivity to sell hardware. The hardware then leads to more software sales. Hardly anyone has a Playstation and an X-Box... its usually one or the the other.
VR HMD's are probably a similar parallel - most will have a Vive, or a Rift, but not both.
Remember the people handling the money are not called 'Palmer Luckey'. They're finance brokers who won't likely have any actual 'buy-in' to the hardawre, never tried VR and possibly never will.
But they will remember the X-Box success and try to replicate it if at all possible.
SO I think we'll see a similar story play out over the next 1-2 years. I think we can expect market segmentation of the HMD space, software-specific support (meaning it'll be more costly for developers to write multiple HMD rendering paths into their games). I will be (pleasantly surprised) if Valve stays as open as it has done so far.
Facebook (and Valve) are interested in VR not for the tech, not for the game's income even. They're big dollars but there's a bigger goal. They want to track your gaze, see what you look at (in-game) and derive marketing data from the VR masses in a way that ads, targeted web-browsing and cookies never could. The further they get into the mind of the consumer, the more effective their advertising will be. This isn't tin-foil hattery; marketing methods have changed radically over the past 10 years, and will continue to change.
They say 'the eyes are the window to the soul'... well, they're about to crack the neck... soon it'll be gaze tracking.