Why do prosecutors reduce some cases and not others?

Transcription: 

Well, prosecutors evaluate their cases just like criminal defense lawyers do. The first thing on their list of things that they evaluate when figuring out how to prosecute or whether to reduce a case is the level of their opposition. Obviously, if they're dealing with me as opposed to a younger attorney, they're going to know I'm going to be able to figure out what's wrong with a case, if the case has to go to trial, I'll try it. Just because you failed the field sobriety test or you've blown over the legal limit doesn't matter to me. I know there are defenses to that case. If you want to go to trial, I'm not going to force you to enter a plea, so prosecutors know that. So they want to win cases. Now, who do you think going to win cases if they tried the case, it ain't going to be me. It's going to be somebody else.