The main legal impediment was the statement of the officer who gives the breath tests. He said the suspect did not appear to be impaired when interviewed. Such information, by law, must be shared with a defendant's lawyer. A dropped case is not unique. Few of the cases taken to the State Attorney's Office go to trial and even fewer result in convictions.
Prosecutors routinely reject cases for insufficient evidence.
There are at least two reasons not to go forward with skimpy evidence. One is political. An elected state attorney would have a record showing a high percentage of cases being lost if he filed cases he had little chance of winning. But he also has a duty not to clutter up scarce court time with such cases. Like anyone else, the suspect received a ticket for running a red light and one for not having his driver's license.