Som ATB-kommentarer i en kommentar till frågan ville inte försämra Beleriand. Det hände just så att Morgoth var så kraftfullt, att förstörelsen av Beleriand var en oundviklig följd av att besegra honom 1 ; Vi berättas i The Silmarillion som:
[S]o great was the fury of those adversaries that the northern regions of the western world were rent asunder, and the sea roared in through many chasms, and there was confusion and great noise; and rivers perished or found new paths, and the valleys were upheaved and the hills trod down; and Sirion was no more.
The Silmarillion III Quenta Silmarillion Chapter 24: "Of the Voyage of Eärendil and the War of Wrath"
Så orsaken till att Ossiriand är rädd är ganska enkel: Morgoth hade inga betydande styrkor i Ossiriand; Faktum är att vi också berättar detta, efter Anglos belägring (betoning av min):
[The Orcs] passed over Gelion with fire and terror and came far into East Beleriand. Maglor joined Maedhros upon Himring; but Caranthir fled and joined the remnant of his people to the scattered folk of the hunters, Amrod and Amras, and they retreated and passed Ramdal in the south. Upon Amon Ereb they maintained a watch and some strength of war, and they had aid of the Green-elves; and the Orcs came not into Ossiriand, nor to Taur-im-Duinath and the wilds of the south.
The Silmarillion III Quenta Silmarillion Chapter 18: "Of the Ruin of Beleriand and the Fall of Fingolfin"
1 Jag diskuterar orsakerna till detta lite mer fullständigt i mitt svar på Varför gjorde Valar överge Moriquendi och män?