It was Regional for a number of reasons. 1: Jerusalem had the unclean birth in the land of Praise/Judah
2: Jerusalem; the BullsEye of The Whole Bible.
3: This IS loooong before the Covenant made In Love by Our Father through the families of Abraham, Isaac and Jacob that didn't go anywhere geographically speaking until the time of Joshua; which is loooooong after the Covenant that began with Abraham and Sarah in the beginning of the 120 flesh year generations.
4: This is a cleansing of Jerusalem of Giants/Geber/Nephilium; the product of beings of flesh, that resulted from flesh humans mixing with fallen angels.
5: There were still Giants/Geber/Nephilium left alive in ALL the neighbouring areas of Judeah, as it is not until after the flood that Israel goes forth to subdue these great walled and evil cities, that they were instructed to destroy EVERY man, woman and CHILD! (would be pretty hard to understand, IF! one has not heard The Father's Word.) as these were the FIRST land blessings that were acquired by Israel; and when studied, known to be for the tribes of Judah and Benjiman.
The other 10 tribes taking The Word geographically from there with Joshua, facing the same issue; dealing with lands infected by Giants/Geber/Nephilium and ignorant of GOD'S Word. This goes on until David eventually. finally cleanses Jerusalem again when he becomes king of both Judah and Israel; BUT! theres still the land blessings of Mannasah and Ephriam, Jacobs/Israel's grandchildren to consider, that have not yet come to pass.
6: There are the land blessings of Essau, Lot and Ishmeal to consider, that have no records in their History that suggests that they went through such an ordeal.
7: A living Dove brought a live Olive Branch to Noah suggesting that there is still life besides whats on the ark.
That's a few reasons anyways.