That's because plants (and this would hold true for other biological photosynthetic systems like algae) aren't very efficient converting solar energy. Solar panels are more efficient - and the sunlight gets converted directly to electricity. The problem is storing that electricity - batteries are expensive and they've reached a ceiling on energy density.
That's where hydrogen comes in. It stores energy based on chemical bonds - not electric charges like a battery - so the energy density is much higher than a battery. With the discovery of a cheap and efficient water-splitting catalyst by Daniel Nocera in 2008, connecting solar panels to an electrolyzer (and from there to a hydrogen storage/fuel cell) is now much more feasible. And it doesn't need to be done on a large scale like a solar farm - instead the energy is generated at the point of use. A distributed system like this means there's no need for a massive infrastructure project and it means that its a viable solution to the energy needs of the developing world. In addition, combusting the hydrogen (generated from dirty water) in the fuel cell produces pure water as a byproduct, suitable for drinking.
Nocera, named one of Time's 100 most influential people earlier this year, explains all this in an expository article in November's Inorganic Chemistry (http://pubs.acs.org/stoken/presspac/presspac/full/10.1021/ic901328v?cookieSet=1) - The Chemistry of Personalized Solar Energy
Not biofuels. Not nuclear. Personalized Solar Energy.