Because the surface pressure and surface temperature permit the existence of liquid water. Earth is unique in solar system in having the right p and T for liquid water. Saturn's moon Titan is the only other body to have a stable liquid at the surface, although there it's methane. read more
The origin of water on Earth, or the reason that there is clearly more liquid water on Earth than on the other rocky planets of the Solar System, is not completely understood. read more
But Earth’s early history, including epochs with high ambient temperatures and no enveloping atmosphere, implies that surface water would have evaporated and drifted back into space. The water we encounter today, it seems, must have been delivered long after Earth formed. read more