There's also the personal ambition of many prosecutors. See Rudy Giuliani and Chris Christie for example, and countless other US Attorneys and District Attorneys and state Attorneys General. One of the ways to enhance your reputation as a prosecutor is to have a stellar (i.e. very high) conviction rate. Plea bargains count as convictions.
Interesting that you should single out two Republicans, both known for being reformers rather than abusers of the system.
Here's what Wikipedia has to say about Christie:
Besides doubling the size of the anticorruption unit for New Jersey,[61] Christie also prosecuted other federal crimes. For example, he obtained convictions of brothel owners who kept Mexican teenagers in slavery as prostitutes, convicted 42 gang members of the Double II Set of various crimes including more than 25 murders, and convicted British trader Hemant Lakhani of trying to sell missiles.[62] Despite claims of entrapment,[63] Lakhani was convicted by jury in April 2005 of attempting to provide material support to terrorists, unlawful brokering of foreign defense articles, and attempting to import merchandise into the U.S. by means of false statements, plus two counts of money laundering. He was sentenced to 47 years in prison.[64]
Giuliani made his name prosecuting the "Five Families" back in the 1980s, the Mafia Commission Trial which helped put away the leaders of the most notorious Mafia families and some of their associates. Giuliani also prosecuted Boesky and Milken, the infamous Wall Street financiers.
There are indeed quite a few very bad players, like U.S. Attorney for Massachusetts Carmen Ortiz, whose notoriously overly zealous attack on the MIT Library hacker Aaron Swartz drove him to commit suicide.
