Dan Herbertson at MMA Fighting has the scoop:

"Towards the end of the negotiations, because of reasons that I am not able to tell you, although the two fighters agreed to fight, it happened that these fighters could not fight. It's not because of money, but if the reasons are something that I cannot tell you then you might be able to guess what they would be. Beause of that, we are not able to have these two fighters fight."

Interestingly, Ricco Rodriguez is never mentioned as having been a potential opponent for Overeem.

Instead, Andrei Arlovski is given as the fighter who was lined up to face Overeem, but the fight was scrapped due to some nebulous non-reasons that Sasahara doesn't go into.

Read the full statement as recorded by Dan here, although it can be summed up as "The fight is off and I'm not telling you why, in a very roundabout Japanese way."