Well why does our solar system has only one sun?
Anyways…. Creation of moon was a coincidence
A space Metroid collided into earth and due to earth is not so gaint that it's gravity can hold much mass single moon was formed with dust and small Metroids.
The EarthMoon is gigantic compared to the planet, a ratio of 1 to 81 whereas for the outer planets it is 1 to 10,000 for the whole moon system.
Extra moons round the Earth would be unstable.