What do we learn from "ha'Lo Es ha'Devarim..."?
Rashi: What caused the fast and eulogy to you - the rebukes that Hashem called. They destroyed you 1 .
Malbim: This will arouse Hashem opposite your actions - what the first Nevi'im called (9), to do Mishpat, Chesed and mercy. Then Hashem will do so towards His creations. Now it is destroyed. It cannot be rebuilt if you will not guard these matters. Something that caused to make something new that did not exist, all the more so it suffices to sustain what already was! E.g. fire can heat cold things - all the more so it can sustain heat, lest the matter cool off!
For you did not heed them. (PF)
Why does it say "bi'Hyos Yerushalayim Yosheves u'Shlevah"?
Radak: While Yerushalayim was serene, I called via the Nevi'im for you to repent from your sins, and the land will not be destroyed. You did not want to hear!
What do we learn from "veha'Negev veha'Shefelah Yoshev"?
Radak: The entire land was serene. It mentions the Negev and low plains, and all the more so the mountains and hills were serene.