The aftermath of a great military victory is usually a time of celebration. However, when the triumph is inseparable from a devastating personal tragedy for the kingdom's leader, reporting the outcome becomes a deeply delicate task. Delivering such a complex message requires a specific kind of messenger. Recognizing the heavy burden of the news, Joab prevents Ahimaaz from running to King David. Although the rebellion has been crushed, the victory is thoroughly entangled with the death of the King's son, Absalom. Therefore, this cannot be treated as a standard report of salvation.
Joab clarifies to Ahimaaz that he is simply not suited to carry such a complicated and painful message. As a good man and a priest to God, Ahimaaz is meant to deliver only joyful tidings that bring comfort and happiness to the listener [אברבנאל, מלבי״ם]. Furthermore, acting as a messenger for the day's events is not his official duty [מלבי״ם]. Because the report will undoubtedly bring sadness rather than joy to the King, it is entirely inappropriate for Ahimaaz to be the one to deliver it [מצודת דוד, ביאור שטיינזלץ].
Instead, Joab assures Ahimaaz that there will be other opportunities. In the future, a complete and purely positive salvation will arise, and Ahimaaz will be able to carry that joyful news to the King [רש״י, רלב״ג, רד״ק, מצודת דוד]. If Ahimaaz's motivation is to praise and thank God for the military success, he will have the chance to do so on other, more fitting occasions [מלבי״ם]. For the present moment, however, Ahimaaz must remain silent because the current victory is fundamentally incomplete in the eyes of the King [רד״ק]. Any attempt to speak of God's kindness in winning the battle would inevitably force the messenger to mention the fate of the primary rebel. Reporting the victory requires reporting Absalom's death, instantly turning a message of triumph into one of profound grief [מלבי״ם].
The central reason for holding Ahimaaz back is rooted in the nature of the prince's death itself. There is an underlying layer of meaning to the event, indicating that the King's son died rightfully and justly. His death was a direct punishment from God for his rebellion and evil actions [רד״ק, מנחת שי]. Yet, even though God's justice was perfectly served, the reality of the situation remains a bitter and harsh tragedy. Such grim tidings are simply not meant for a good and holy man like Ahimaaz to deliver [מלבי״ם].