Another possible answer is that, after the massive and collective effect that the elixir caused in the population, you can argue that they reached the zenith of extreme passion that shut down the basics of humanity, and blinded them to everything else. The scene of the massive orgy is meant to demonstrate the hyperbolic nature of the entire story: All is blown and exaggerated to bring out the intensity of the message.

The elixir that came out when the perfume bottle was open basically had them explode in some form of ecstatic unconsciousness, and left them different than what they were.  Being that their lives and consciousness were eradicated by such a radical force of nature, they became numb to anything else, perhaps the result of a massive "shut down and restart" for lack of better words.

The answer to this question can be found in looking at some basic traits of human nature.  In the story, the murders struck fear and terror into the hearts of the townspeople, and was the constant talk and obsession of them while they were occurring.  However, after the threat faded and the murders stopped, the people eventually forgot about what had happened, and went on with their daily lives.

Their lack of remembrance can be attributed to the fact that often, as humans, we do forget about tragic occurrences in our lives.  And, it isn't necessarily "forgetting," but a lack of thinking about it as much.  Take for example 9/11.  Right after that happened, we were terrified, shocked, angry, and fearful.  However, with passing time, many of us stopped thinking about it at all, and put those feelings behind us.  This trait of humans to leave the past behind us can be seen in two different ways:  we are shallow, emotionally stunted people who don't feel anything unless threatened immediately by it, OR, as a necessary psychological coping technique to help us to move on with our lives without being paralyzed by fear every day.  Either one of these interpretations can be applied to the people of Grasse.  They were either busy and shallow and forgot about it because the threat was gone, OR, they needed to bury the events in order to move on with their lives in a healthy way.  I hope that those thoughts helped; good luck!