It has to do with their sales approach and technology. Last time I checked they are selling a lot directly to consumers with local slaes reps. That is expensive. Offering leasing agreements is just adding to their financing needs and cost.

Installation requires a local entity, either in-house or a third party, to plan it, visit the premises and so on. Offering a fixed rate up front (some companies in Germany do that, not sure whether Solar City is doing the same) carries risk for bith parties. In a nutshell, sales is expensive and capital intensive.

Regarding tech, solar modules are a commodity by now, they get cheaper basically every quarter. And more efficient ever year at least. So on that front there is not a lot to gain. Also, full grid independence is still not achievable for reasonable prices. Smart grids are better solution, inverter producers and utilities are in a better position than the company produucing and installing solar modules.

The technological curve results in guaranteed obsolescence. For solar roofs that means either to keep production for older models up or reliability high enough for the full life time of a roof.

Just a couple of thoughts.