Never explained, nor even hinted at. Oh, and btw, the moon doppelgangers aren't the only ones. There are also some earthly doppelgangers, but those are apparently friendly(and not mentioned much except for the intro thingy.
As for fund raising, they are doing it the same way the Mars One guys were planning it. Try to make money of the press conference.
It's not a bad book perse, but it would have been improved a lot if the writer had just dropped the entire moon thingy. Really, it was nice till they launched towards the moon, and from that point on you were just waiting for the astronauts to be murdered. Because bloody hell they were bad at their job. (Another nice thingy, the only weapon on the mission was a single old sixshot revolver, given to the only astronaut they had bothered to brief about the dangers.)
Edit:
Also, there's something weird with the translation.
The English/original (according to wikipedia synopss) version ends with a report of a future European mission that found the bodies, but not the doppelgangers.
The version I read ended with a diary page thingy from a Japanese girl, 2 years after the doppelgangers have destroyed the entire human resource.
Edit 2: I also haven't mentioned any of the completely strange stuff. (Ie, where the book doesn't even attempt to justifify). Or really, most of the science erros.