Ultimately the reason why he did it is because he could and I blame the nonexistent fact checking at Spiegel for this. But on a more personal level Relotius said he couldn’t stand the pressure of not delivering a great story (he won multiple awards first). I would also blame these awards for beeing to trusting, somebody like him should never have reached this level of fame and reputation.
I never really read his stuff, but I remember reading one story a long time ago that had someone start singing a song and I remembered wondering if this really happended or if I am in the realm of phantasy there. Now I know.
The problem is incentives, always