True service to God has always been about moral integrity rather than empty rituals. Long before the destruction of the First Temple, prophets warned the people that God desires justice and kindness over outward displays of mourning.
The primary approach among commentators is that God expresses a profound wonder regarding the people's actions, asking who even requested them to fast. God does not need fasts. Since He does not eat or drink, depriving oneself of food is not the way to walk in His paths. Instead, true closeness to God and the ultimate redemption depend entirely on refining one's character and practicing true justice, kindness, and mercy [אברבנאל].
Alternatively, this message serves as a sharp reminder of what caused the exile in the first place. The very prophetic teachings the people once ignored are what ultimately brought about the destruction of the land. By refusing to listen, they brought the tragedy upon themselves, forcing them into a state of fasting and weeping [רש״י, רד״ק]. Building on this understanding, there is a powerful logical conclusion regarding the construction of the Second Temple. If obeying the prophets was essential to keeping Jerusalem safe and peaceful while it stood, it is absolutely required in order to rebuild the city from its ruins [מלבי״ם].
To emphasize what was lost, a contrast is drawn with the era before the destruction. Back then, Jerusalem was fully inhabited, resting in glory, quiet, and complete safety [מצודת דוד, רד״ק, ביאור שטיינזלץ]. The surrounding cities shared in this absolute peace. This security extended across the entire land, reaching even the southern region and the lowlands [מצודת ציון]. These areas sat quietly and safely in their proper state [רד״ק, אברבנאל]. The specific focus on these lower, open regions highlights a broader reality. If the most vulnerable, unfortified parts of the land enjoyed total peace, the fortified mountains and hills certainly experienced full security and prosperity [רד״ק].