This isn't correct. There were/are a lot of inside out VR headsets. Oculus just happens to have a brand, a solid marketing presence, a solid product, the appearance of longevity etc.
Facebook might have helped but from a purely technical perspective, Facebook wasn't the only path.
The Lenovo Mirage Solo had 6DOF tracking with the Daydream controller. It had an experimental 6DOF controller add on but Google killed Daydream before that shipped. Inside out PC headsets exist with the camera setup for 6DOF inside out.
Currently the Quest is the only Android based (and thus fully stand alone) headset that's wasn't Daydream branded and wasn't killed when Google mothballed their VR support. There's less now then there were, or would have been if Google stuck with it.
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Facebook might have helped but from a purely technical perspective, Facebook wasn't the only path.