It’s not difficult to guess: they got EA’d. IBM’d. FaceBook’d. Their startup got bought up, hollowed out, and dissolved. All in the name of killing off competition and padding staff rolls.
I’ve found many startups are merely “investments” by some entrepreneur that were intended from inception, whether explicitly or not, to be grown to a sufficiently negotiable state and sold to the biggest buyer. That’s not to say that big tech companies don’t buy-out their competition, but many startups also dream of being bought-out.