?
Because "Spaceport America" doesn't want rockets exploding all over it, and be the starting point of the next great tragedy in space.
quantumclaustrophobe
Potentially, they have stages that could fall & crash after a launch... Spaceport America is located such that it may be that falling stages could be a danger to people downrange.
ANDRE L
Because the kinds of launches they do require a long distance of water to the east of the launch site.
?
maybe they dont want to for some reason
skeptik
Because they still prefer to do most of their testing over water. There isn't any available in the New Mexico desert.
The Spaceport isn't hurting from the lack. They have several other customers who do use the facility. Virgin Galactic being the most famous.
I suspect Branson doesn't want Musk to get too close to his trade secrets. And vice-versa.