Because Tesla won’t let other cars use Tesla supercharger network. Imagine different gas pumps existed for Toyota because Toyota has more cars than any other company. Toyota owners will have to only go to Toyota pumps while other brands share gas pumps.
Tesla saw a need, so they built a system, knowing it was the only way to get BEVs adopted anytime soon. And they’re right - without the supercharger network, I’d still have my Volt, and I’m sure I’m not the only one.
Elon has said on various occasions that they’d be open to partners in the network, provided the partners paid to support and expand the network in proportion to their usage. So far no one has taken them up on it.