Prosecutors have discretion to not bring charges. Wayte v. United States 470 U.S. 598 (1985), Bordenkircher v. Hayes 434 U.S. 357 (1978)
The Department of Justice says:
The reasoned exercise of prosecutorial discretion is essential to the fair, effective, and even-handed administration of the federal criminal laws. Decisions about whether to initiate charges, what charges and enhancements to pursue, when to accept a negotiated plea, and how to advocate at sentencing, are among the most fundamental duties of federal prosecutors.
The US Attorney's Manual says:
The prosecutor's broad discretion in such areas as initiating or foregoing prosecutions, selecting or recommending specific charges, and terminating prosecutions by accepting guilty pleas has been recognized on numerous occasions by the courts. See, e.g., Oyler v. Boles, 368 U.S. 448 (1962); Newman v. United States, 382 F.2d 479 (D.C. Cir. 1967); Powell v. Ratzenbach, 359 F.2d 234 (D.C. Cir. 1965), cert. denied, 384 U.S. 906 (1966). This discretion exists by virtue of his/her status as a member of the Executive Branch, which is charged under the Constitution with ensuring that the laws of the United States be "faithfully executed."
As of April 28, 2016, 35 bankers had been sent to prison for fraud and other financial crimes relating to the 2008-and-onward financial crisis.