First off, Japan is space starved and property prices around it's population centers is staggering. Also, 80% of the country is mountainous.
Second of all, the irradiation in space is higher, about 1.3kw per square meter vs 1kw at the equator.
If you count in the fact that the sun is shining 24 hours a day 365 days a year.. versus the estimated 6 hours a day of good sunshine down on earth, you end up with roughly 5x the light per square meter of collector per hour per day averaged.
There's also no dust to mire the panels in space, or atleast not dust as we know it on earth. And if concentrating solar is used, like in the mirror design, high efficency solar panels can be used, again resulting in a 1.5x-2x boost in efficency per square meter of collector.
Cost wise though it'd probably be cheaper to fill every house's roof with solar panels, before going the space based route first. Economics is likely to be much better, unless they find a way to launch massive unfurling mylar mirrors into space, get them to play nice, and do it cheaply.
This all being said, if there's any way you could use local asteroids to start fabricating SPS's in orbit, then the economics change entirely and it's not so easy to say how it will play out.