As has been said earlier, Ashok had demobilized his army and turned them into monk missionaries to carry the gospel of Four Noble Truths and they moved about precisely in that orderly and disciplined manner. It was the serendipity of this step that made the monk missionaries so successful in their mission.
The monk missionaries who went East and found vast empty lands, they encouraged the population to migrate en masse and settle down there to establish a new kingdom in the name of the Buddha. Those monk missionaries who went West however, encountered a very different scenario: there were well organized and sophisticated society already flourishing there, it was Greco-Roman world.
Amidst this world, the monk-missionaries had established their own cave monastery, now known to us as Petra, simply meaning Rock as has been mentioned earlier; it had been abandoned some two millennia ago. The successors of Ashok the Great were unable to hold on to their empire as local chiefs rose up who championed the cause of Hinduism and the Empire got fragmented and disappeared. Buddhism without any support and patronage just withered away and Petra suffered the consequence.
Ashok the Great was well informed of the situation for he has named the Hellenistic King Antiochus (215-164 B.C.) of Seleucid dynasty as his westerly neighbor. Ashokan monk missionaries were familiar with the Judeo-Hellenistic world becomes apparent from the edict they have left engraved bilingually in Greek and Aramaic.
Nearer home, all Ashokan edicts are written in Pali, some fifty of them still survive; they are one of the oldest form of writing in evidence, dated 3rd Century B.C..
Photo Greco-Aramaic Bilingual Edict of Ashok, circa 3rd Century B.C.: Courtesy Wikipedia