We don't really know. One possibility is that the orbit of the Earth is the closest to the sun that water/ice can exist so that if a planet forms here and farther out in the solar system, it can get quite a load of water and water ice. The satellites of the major planets are solid water ice most likely...entire worlds made of water! Mars lost most of its watr because it just couldn't hold on to an atmosphere and the water was photo-dissassociated by UV light from the sun.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.