The situation might be more complicated. If I was a rights holder for movies that Netflix wanted to stream (and sufficiently Machiavellian), I might negotiate reduced availability of DVDs of my movies to start transitioning everything to a streaming model.
To be clear, Netflix may have the DVDs but they may be bound by a contract clause (that they agreed to) restricting them from offering the DVDs for rent.
That said, maybe it's just Netflix reducing costs by dropping obscure inventory. I don't know what the relative costs and profit are of the streaming vs. DVD businesses that they have, but the costs of that physical inventory are not negligible.
Back in the day when Best Buy opened a new store they stocked the CD and DVD sections with incredible amounts of diverse inventory and then let it sell off - restocking only the mainstream stuff. I assume the goal was to get customers with a front-end flourish. That's probably what Netflix did.