Actually I think it's the scale of it. Governments, charity/foundations, universities, and a lot of large companies are pumping billions of dollars into those fields. You can walk around blindfolded and you'll run into 5 universities building new 'green energy research' and 'biomedical research' labs. A single billionaire can't really make a dent in that. Bill Gates even makes that point when he talks about education - his entire wealth is a drop in that bucket.
But space is a market where a billionare can make a difference. Until now the only game in town was NASA and its contractors. And they are not very efficient. SpaceX in a nutshell is proving what modern managment techniques and modern engineering tools can do to the cost structure of NASA-like technology. That's not that far out there. The payoff is pretty likely to happen (contracts from NASA, contracts to launch private satellites). The payoff for nanotechnology is way way further out and far more nebulus.
But space is a market where a billionare can make a difference. Until now the only game in town was NASA and its contractors. And they are not very efficient. SpaceX in a nutshell is proving what modern managment techniques and modern engineering tools can do to the cost structure of NASA-like technology. That's not that far out there. The payoff is pretty likely to happen (contracts from NASA, contracts to launch private satellites). The payoff for nanotechnology is way way further out and far more nebulus.