What is the significance of the fact that it was the Nochri nations who made the current observation?
Oznayim la'Torah: It is interesting that, even as Yisrael were busy trying to cast off the yoke of Hashem and to adopt their lifestyle, they - the nations - realized that Hashem was punishing them for abandoning their G-d. 1
See Oznayim la'Torah DH 'Ve'amru Kol ha'Goyim #3 and on Pasuk 25,.who elaborates.
What caused the nations to become shocked?
Seforno: It was the sudden influx of exiles that appeared in their land that caused them to wonder what happened.
Hadar Zekenim (in Pasuk 20) and Rosh: They wonder why Yisrael were punished for murder and Arayos whilst other nations are guilty of the same sins, yet their lands are not laid desolate in this way! 1
Lev Eliyahu (Sh'mos p.84): Because their punishment defied the laws of nature - in that all of Eretz Yisrael became filled with sulfur and salt, and that even grass, which normally grows by itself, should stop growing.
Oznayim la'Torah: Because with any other nation they would have simply attributed it to the laws of nature - today this nation is up and that one is down; tomorrow their roles are reversed. But they could not fathom how Eretz Yisrael, which until recently, was renowned as a land flowing with milk and honey had suddenly become totally devastated..
Hadar Zekenim and Rosh: The reason for this is because, as opposed to Yisrael who entered the B'ris of Avraham, and later abandoned it, other nations did not enter into a B'ris, as the Navi writes in Yechezkel 20:32 & 33 "ve'ha'Olah al Ruchachem
