They left the whole kingdom. They were the most prosperous on the earth. The whole city was made of gold. The temples, the palaces. They have well-defined city structures and city organization was similiar to that of Mohen-jo-Daro and Harrapa. Civil facilities like rest houses for travellers across borders. Everything they left. They boldly faced centuries of attacks and they suddenly, they withdraw. The people of Angkorwat suddenly disappear. why?
Ankorwat was then the capital of Khmer empire. The Empire which regained its strength in 802 AD and keep prospering for about 5 centuries. Ankorwat temple was built by Suryavarman II in the early 12th century. There is one peculiar thing about this temple which is noticed. Unlike other temples, this temple directs to the west. Some scholars debate that this temple was dedicated to Vishnu, the protector who is situated somewhere in the West(I think its either somewhere in mid-pacific or mid-altantic, either he could be some whre in mid-atlantic ridge or in Mauna Kea, Hawaii ). The earlier rulers were Hindu and later on, they were buddhist.
In 1010 AD, SuryaVarman I became the king and and declared ‘Buddhism’ as state religion. However, he allowed the people to follow Hinduism. People loved their gods, so they continued worshipping them. But the king lost the faith in old gods, they were uprising from everywhere. SuryaVarman II reunited the whole empire and built Ankorwat temple dedicated to Vishnu, The Protector. Even Jayavarman VII who was buddhist expressed his solidarity to Hindu Gods. He is still regarded as the best king Cambodia ever had.( and the worst were Khmer Rouge! They ruined Cambodia)But later on, Buddhism spread and the temples were converted to dedicate to Buddha. Khmer rulers became more tolerant as they accepted Buddhist philosophy and their attackers became more aggressive. They finally abandon everything and left to somewhere. Where did they go?? I mean how can do they leave their own city like this. Some people suggest they formed a new kingdom in south. But again, it was not so prosperous Why??
The Harrapan Civilization was much more bigger and extensive than Khmer empire. They too have planned cities and structures which would keep all the present seven wonders out of the race. Why do they left? They too faced attackers from West. But How could one leave such a prosperous civilization?
Although the exact history remains murky, some scholars suggest that they were attacked by Aryans who destroyed the whole civilization, adopted the loser’s religion and instead of either returning to their own land or imposing their rule on the conquered land, they cleared jungles of Ganga and establish a new civilization. This is utter non-sense and completely unbelievable ! (UN commented on it by sayng that indian historians have committed a heinous crime against india’s heritage and culture.)
I totally disagree with British time Theory that Aryans invaded and made Dravidians their slaves. I believe From the ancient times, there has been an agreement not to cross Vindhyas and attack Dravidians.(Though some violated it!) The relation was like present USA -Canada relationship.The theory of attacks on Jain and Buddhist temples and loot them is all bogus and are nothing more than Machiavellian plans by British scholars to disrupt Indian unity at that time. The Bloody communists who denied any kind of religion further deformed our history. (I hate indian communists! They are neo-autocrats who have destroyed West Bengal).
The answer lies in nature. The nature took a toll on these civilizations. There was a drought in 15th century which forced khmer people to withdraw. No food, no life! The Dutch left and settled in the region which is more turbulent just to get the fertile land. The Indus people left b’coz there were huge tsunamis which arised due to bombardment of asteroids in Arabian sea. These tsunamis might have been as big as 100 feet and wiped out the whole civilization. The salt waters made the land infertile. The Rann of Kutch is the proof. They are forced to move to the plains of Ganga and start a new civilization all together.