For me, cost.
I pay an average of $70 a month for electricity or about $840 a year.
A home solar system in the 5 kW to 7 kW seems to be about $30k. Even with a 1/3 Federal tax credit, $20k for me doesn't make sense at what electricity costs me. I do live in a place with relatively inexpensive electricity. In another state (and one with more sun than where I am), that could change.
Interesting we were just in Belize. 17 deg about North of the equator, sun pretty much all year around. Didn't see a single solar panel installation. Didn't see any solar hot water heating systems (in contrast, when we were in Turkey years ago, pretty much every house in the interior had solar water heating systems).
Poor math? I know I've argued with people who say they save $1,000 a year on a $20,000 system and therefore they are making 5% on their money. Wrong. It takes 20 years just to recoup your initial investment, you haven't made a dime at that point.
You give me $20,000 and I'll pay you back $1,000 a year and hey, you are making 5%. Try beating that at a bank.