It may or may not be because of lack of evidence.  There are a couple of different possibilities.  The first is lack of evidence.  The prosecutor could have reviewed the case and determined that there was not enough evidence to go forward.  The second possibility is that the prosecution has found a way to enhance your history and may file it later as a felony charge. A third possibiity is that they are waiting on additional evidence before they file the DUI.  Other evidence could be in the form of medical records, prior judgments, or witness statements.  Many prosecutors don't like filing a charge until they have all of the relevant information.  When you go to court, you need to inquire about the DUI before you enter a plea on the other charge.  By admitting to part of the offenses that day, you may be giving the state additional concessions to use against you at a later date.