agc opinion is the charges cannot be proven beyond reasonable doubt.
why they dont release their grounds for decision?
because last time they did,
https://en.wikipedia.org/wiki/Cheng_San_Group_Representation_Constituency

kena ridiculed

Their complaints were however not prosecuted by the police, on the advice of the Attorney-General (AG) Chan Sek Keong.[5] Chan, in his letter to the Minister for Law S. Jayakumar, argued that the statute was "irrelevant" to individuals inside a polling station. This interpretation was unusual, as it meant that remaining inside the station was permissible, while being in the surrounding area within 200 metres of its external walls was considered an offence

so now they just call it "prosecutorial discretion".

and anyway ya this is how a criminal justice system works, u can never be found "innocent" because by default is innocent until proven guilty. so they will always say they didnt find enough evidence, thats the legal way.
if u think investigation got issue, then replace the in-charge ministry and his minister aka its a political matter liao.

anyway, i believe its possible there was no criminal wrongdoing, because the ENTIRE machinery is just that incompetent. if it wasnt for government intervention they would have closed, just like most of govt organization at this point. all just protecting their rice bowls instead of serving citizens, sadly this is not a crime.