The short answer is being as close as possible to the equator and able to launch over an unpopulated area (like an ocean) is more important than altitude. Ther

The short answer is being as close as possible to the equator and able to launch over an unpopulated area (like an ocean) is more important than altitude. There might be some benefit to launching off of a really tall mountain, since it would make your first stage engine more efficient, but not enough to justify the extra logistics.