For short, after the fall of Khmer Empire, it kept being invaded by Thailand and Vietnam and everybody was so busy trying to not die so they didnt really have time to write down history of being invaded and being vassals of Thailand or Vietnam anymore. That or the habit of Vietnam and Thailand to burn down everything and looting so hard that barely any record from these times survived. After a while, French came and you know the rest.

There is a reason why Cambodia is extremely salty against Thailand and still somewhat salty against Vietnam.

I always thought the same thing. When it comes to Cambodian history all you really hear is the Khmer Empire and the Khmer Rouge. Between those two periods in time I have no idea what it was like.

The Khmer Empire was pretty much a declining state by the time Angkor was sacked in 1431. The surviving court fled south but the Khmer people retained a semblance of identity and autonomy for a time. The sacking reduced them to a minor power player until they became a protectorate under French rule. In the meanwhile, they were a pseudo-vassal state to Siam. King Norodom was wise in asking the French for protection. I don't think present day Cambodia would exist as a state had the French not intervened.

The issue with it's successor state (and at periods even during the Khmer Empire) that kept it from stabilizing was the constant infighting between factions of competing royal houses. We see this recently happen in the 1970 coup of Prince Sihanouk by Prince Sirak Matak with dire consequences. So rather than dealing with external threats such as the Vietnamese and Thais (who were forced out of China by the Mongols), Cambodia was in constant political strife which did not help it's position.

Cambodia sits on prime real estate on the Indochinese peninsula so it naturally became a prime target for conquest and annexation from both Siam and Vietnam. Though initially both nations wanted Cambodia wholly for themselves. Siam being culturally similar to Cambodia viewed it as a part of it's political sphere of influence. Vietnam viewed Cambodia as a barbaric state to be conquered. This contributed to more infighting between Cambodian factions that were either pro-Siamese, pro-Vietnamese and a powerless pro-Khmer.

The situation for Cambodia further deteriorates when the Mekong Delta (Kampuchea Krom) is annexed by the Vietnamese in the mid 18th century. This was a result of the marriage between a Khmer King and Vietnamese princess. The union of these two royal houses leads into an agreement in which Vietnamese settlers would be allowed to settle into Kampuchea Krom. Later refugees arriving as a result of the civil war in Vietnam made the native Cambodian population a minority. Interestingly enough, the Mekong Delta and Saigon (present day city of Ho Chi Minh) is where the French would take hold and eventually annex both Laos and Cambodia forming the colony of French Indochina. Siam does the same thing in it's annexation of Battambang and Siem Reap (essentially all of northwestern Cambodia) in 1795. Siam would not relinquish control of these territories until 1907 under French pressure. They actually seize it back momentarily during WWII.

Record keeping was maintained but constant warring lead to their destruction during periods of political strife. While during the Khmer Empire, stone inscriptions gave us valuable insight, that ceased with the fall of the empire. Most records were kept on perishable materials like palm leaves. So a lot of information of the Dark Ages of Cambodia come from Thai and Vietnamese written sources along with the oral pass down from Cambodia sources. So between the Fall of Angkor to the Khmer Rouge, Cambodians retained their own identity despite all the odds; certainly a very resilient group of people.