The Recabites had kept the rules their ancestors gave them−even to the point of refusing hospitality (no small thing in that culture). Jeremiah compared them Judah (and Israel), and how they had abandoned almost everything their ancestors had agreed to with God. The contrast was used to show Judah where they had gone wrong, and why the impending disasters would happen:
Jeremiah 35:17 Therefore, this is what the Lord God of Heaven’s Armies, the God of Israel, says: ‘Because you refuse to listen or answer when I call, I will send upon Judah and Jerusalem all the disasters I have threatened.’
That Bible blog 