They don't get into the "why" of the price change. A few years ago I read something that said there were two principle reasons:
1. China overbuilt capacity and so there was a glut of panels. The question was whether or not that meant a short-term or long-term fall in prices, and it appears it is long-term
2. Standardization. They went to a jobsite with an installer team and found they had only one hand tool, a special tool that had all the wrench sizes needed to assemble a panel array. The installer said that what was a 2-3 day job a few years before, was their morning install before an afternoon install. Obviously that's residential, not commercial like in the article. Still, I bet there are similar process efficiencies on the commercial end.