Because the Kingdom of Judah had seen the results of Israel's idolatry—had witnessed the catastrophe of her fall and mass deportation, but had refused to repent—God judges that "backsliding Israel has shown herself more righteous than treacherous Judah" (verse 11).
God, through a number of prophets, warns Judah not to follow Israel's course. For example, Hosea, using harlotry as an analogy for idolatry, pleads, "Though you, Israel, play the harlot, let not Judah offend" (Hosea 4:15).
With a few exceptions, notably Hezekiah and Josiah, the kings of Judah were more corrupt than their counterparts in the north. Israel set the pace into idolatry, and Judah enthusiastically followed. "Israel and Ephraim stumble in their iniquity; Judah also stumbles with them" (Hosea 5:5).
Searching for Israel (Part Six): Israel Is Fallen, Is Fallen
Other Forerunner Commentary entries containing Jeremiah 3:7: