NY Times (from 2014):

But Stephanie Yonekura, the acting United States attorney for the Central District of California, which includes Los Angeles, has had lingering questions about the litigation because of arguments raised by lawyers for Mr. Mozilo and other potential defendants, said the people briefed on the matter, who were not authorized to speak publicly about a current investigation.

One argument raised by lawyers for Mr. Mozilo and some of the other defendants is that a civil fraud lawsuit would duplicate the efforts of the Securities and Exchange Commission, which sued Mr. Mozilo and two other former Countrywide executives in 2009. On the eve of the trial in 2010, Mr. Mozilo and the other defendants reached a settlement with the agency that required the mortgage financier to pay $67.5 million in fines and restitution. In settling that securities fraud and insider trading case, Mr. Mozilo and the two other former Countrywide officials, David Sambol and Eric Sieracki, neither admitted nor denied liability.

In criminal law, a person generally cannot be charged with the same crime twice. There is nothing comparable in civil law, but the people briefed on the matter said lawyers for the defendants had claimed it would be unjust for two government agencies to bring two similar civil enforcement actions so many years apart.

I don't know enough about this stuff, but it seems likely that prosecution would have been difficult. The Feds have gone after Mozilo (see above) and Countrywide for lots of stuff so it's not like they've been passive in suing them when they think they have a decent case.

FWIW.

Cheers,
Scott.