Because those big red Tesla signs will also prevent any other brand from joining Tesla's Supercharger network.
Can you see BMW or Audi telling its customers: "Yes, you can charge at Tesla's supercharger network"?
What will never happen. If Tesla _really_ wants to open up it's network, they need a separate name for their charging network that isn't a direct link to Tesla as a brand.
For now there is no reason why Tesla would do this. It is one of their biggest advantages, and they churn out cars faster then they roll out Superchargers, so every Supercharger is seeing increased used already. no need to add cars from another brand at this time.