The tax amnesty was supposed to be a big deal for regularizing cash from the black market and raise 2.5 billion in tax. Turns out that most fraudsters weren’t stupid enough to take the government up on its offer. My guess is that after talking to a lawyer, they realized that it was probably smarter to wait out the four year statute of limitations and keep the money. After all, some of the non-tax related offenses that led to the illegal bonanza might still be prosecutable.
In the future, the government might have more luck on this kind of thing since the new asset declaration law effectively remove the statute of limitations on undeclared foreign assets (FBAR comes to Spain!).