For the past decade, the solar industry has primarily sold residential solar projects through high-touch door-to-door or telemarketing channels. Companies have to push sales to customers rather than rely on customers to pull demand, and that's caused costs for sales and marketing to rise.
There are a lot of reasons solar sales haven't been easier, but a big one is that the process is just too complicated. Someone has to tell you what the solar installation will look like, how much energy it will produce, how much money it will cost, and how big savings will be, and some of those steps can take days or weeks. It's not like going online to order a pair of socks, or even shopping for a car -- rooftop solar is a complicated process, and no one has found a great solution for pushing sales online.
Image source: SunPower.