Rolls-Royce gets $6M to develop its ambitious nuclear space reactor. Why not solar power?

Apparently, solar panels deliver not enough energy supply in space! Is the same maybe not true on Earth?

"... According to Rolls-Royce, the whole system design will be completed in 18 months with the help of academic collaborators at the University of Oxford and Bangor University, with the first orbital test of the reactor slated for before the end of the decade and testing of a Moon version a few years later. 
The purpose of the Micro Reactor is to create an energy dense, reliable, and extremely portable power source for not only long-term exploration and scientific missions on the Moon and in deep space, but as a marketable British system for commercial customers for both space and terrestrial applications. ..."

Rolls-Royce gets $6M to develop its ambitious nuclear space reactor Rolls-Royce has received an additional £4.8 million (US$6.2 million) in funding from the UK Space Agency (UKSA) to develop key technology for a nuclear micro-reactor that could one day power lunar bases and spacecraft propulsion.