Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
Prosecutors have limited resources so that they cannot possibly pursue every single case they are presented. Although I am not privy to their decision making process, I imagine they select cases based on their strength and merit, and the potential benefit for the public.
The prosecutor is supposed to pursue the cases which benefit the public the most. Note I stated "supposed to." Prosecutors are in a political field. Every case they win is another step up the political ladder, every case they lose is a step down. If a prosecutor can't win a case, they won't prosecute it, regardless of benefit to the public. The only exception will be a high profile case with a lot of press coverage (OJ, Casey Anthony, etc.) So either a case has to be on the news, or it has to be a "slam dunk" for a prosecutor to try the case. Otherwise, they will try to plead it out (so they still get a conviction for a lesser charge for their "stats") or drop it entirely.
Disclaimer: This answer does not constitute legal advice, and should not be relied on. Each state has different laws, and each situation is fact specific. Without an in depth consultation and analysis of all relevant facts and evidence, it is impossible to fully evaluate a legal problem. This answer is not intended to, and does not, create an attorney-client relationship.
Prosecutors usually determine whether to file a case based on whether they think they can prove beyond a reasonable doubt that the defendant is guilty of an offense. In the prosecutor's office I worked in (OC) this was primarily what was done. Political considerations were not as influential though I'm sure that happens. I have seen many cases so lame they limped on both legs... but were still filed. Some DAs are smarter than others, so they may reject cases that would be filed by a colleague with less (or more) wisdom. They are, after all, human.
No attorney-client relationship is established between this lawyer and the originator of the question. This answer is provided for informational purposes only and is provided purely to assist the questioner in determining whether to consult with an attorney to obtain legal advice specific to their matter.