Why was the Freeling house the only one being haunted?

It's not exactly explained in the film, but it's possible that the haunting only started at the Freelings and had later spread to other houses. Some points to consider in support of this theory:

• It's established that the spirits were especially drawn to Carol Anne, which explains why they haunted the Freelings first. From the IMDb FAQ:

It could simply be that the poltergeists found that the strongest concentration of spiritual or dimensional energy was located there and they were able to use it to create the portal into our world.

Carol Anne's strong lifeforce is luminescent, and the ghosts see her as a reminder of the lives they once lived. [...] Carol Anne was born in the Freelings' house, [...] so it's possible that Carol Anne has a special connection with the ghosts.

• In Spielberg's original treatment (then called Nighttime), the haunting spreads to other houses:

Within a short period of time, the Freeling neighbors have problems of their own. Screams and panic indicate that like a contagious virus, the haunting has spread [...] Beyond causing frightened residents to harm themselves while fleeing ghostly manifestations, ghost fires are set. [...] Each haunting, in each home, is signaled by the television.

• And it seems Spielberg's original intention in the treatment materialized in the sequel Poltergeist II, where Cuesta Verde is evacuated in the beginning and excavated for its paranormal properties. It's doubtful a whole neighborhood was evacuated just to play it safe, but even if it was, it shows that the other houses were at least in danger of being haunted.