"One of the main problems is the signal to noise ratio over high voltage lines, along with data signals being destroyed by substations and transformer boxes along the way. Main.net gets around the problems with some unique solutions. First they don't rely on the noisy high voltage lines, but instead use existing fiber optic networks and inject the signal downstream on medium voltage lines. To deal with data loss problems they use a CDMA protocol along with repeaters to keep the data flowing."
Debate still exists whether the system is or isn't viable in the US. For example, read this.
More here, and here.