What is the wisdom behind the Prophet’s, may Allah bless him and grant him peace, saying, "When you hear the Iqamah," instead of "When you hear the Adhan" in the aforementioned Hadith, given that walking during the Iqamah may lead to a decrease in reward?

It is recommended to walk to prayer with tranquility and dignity, whether it is at the time of the iqama (call to commence prayer) or before it. The iqama was specifically mentioned in the hadith because those who go to the mosque at that time often rush to catch the opening takbir or a raka'ah. As for those who go before the iqama, they do not rush because they are assured of catching the entire prayer, and they are also required to maintain tranquility. The meaning of the hadith is not to delay going until the time of the iqama and then walk with tranquility.