Because the government won't support renewable energy research. They give tax breaks and grants to big oil companies, but they don't really do the same for renewable energy companies.
Obama and Al Gore may be for renewable energy, limiting emissions and mitigating the damages of global warming, but they face huge opposition in the House and Senate, especially by republicans who either don't believe in global warming or don't think it effects them or benefit by not using green technology (i.e. they are paid by oil and coal companies to keep bills and laws that would benefit renewable technology from passing).
As long as oil and gas and methane and coal are a cheaper option, until the US government passes laws, give grants to renewable energy and proclaims their support, solar energy will not start to take off in the US until we get to the point where we are almost out of fossil fuels and have no choice but to switch.
Plus there are a lot of issues with solar energy that still need to be worked out before it is a viable replacement for fossil fuels. It needs to be more efficient, cheaper, and we need to find a way to store solar energy and send it over long distances like we do with fossil fuels. However, scientists cannot find viable solutions to those problems if the government won't support the research.