In the 1960s, the ascendant Khmer Rouge forged an alliance with ethnic minorities in Ratanakiri, exploiting Khmer Loeu resentment of the central government. The Communist Party of Kampuchea headquarters was moved to Ratanakiri in 1966, and hundreds of Khmer Loeu joined CPK units. During this period, there was also extensive Vietnamese activity in Ratanakiri. Vietnamese communists had operated in Ratanakiri since the 1940s; at a June 1969 press conference, Sihanouk said that Ratanakiri was "practically North Vietnamese territory". Between March 1969 and May 1970, the United States undertook a massive covert bombing campaign in the region, aiming to disrupt sanctuaries for communist Vietnamese troops. Villagers were forced outside of main towns to escape the bombings, foraging for food and living on the run with the Khmer Rouge. In June 1970, the central government withdrew its troops from Ratanakiri, abandoning the area to Khmer Rouge control. The Khmer Rouge regime, which had not initially been harsh in Ratanakiri, became increasingly oppressive.  The Khmer Loeu were forbidden from speaking their native languages or practicing their traditional customs and religion, which were seen as incompatible with communism. Communal living became compulsory, and the province's few schools were closed. Purges of ethnic minorities increased in frequency, and thousands of refugees fled to Vietnam and Laos. Preliminary studies indicate that bodies accounting for approximately 5% of Ratanakiri's residents were deposited in mass graves, a significantly lower rate than elsewhere in Cambodia.After the Vietnamese defeated the Khmer Rouge in 1979, government policy toward Ratanakiri became one of benign neglect. The Khmer Loeu were permitted to return to their traditional livelihoods, but the government provided little infrastructure in the province.  Under the Vietnamese, there was little contact between the provincial government and many local communities. Long after the fall of the Khmer Rouge regime, however, Khmer Rouge rebels remained in the forests of Ratanakiri. Rebels largely surrendered their arms in the 1990s, though attacks along provincial roads continued until 2002.Ratanakiri's recent history has been characterized by development and attendant challenges to traditional ways of life. The national government has built roads, encouraged tourism and agriculture, and facilitated rapid immigration of lowland Khmers into Ratanakiri. Road improvements and political stability have increased land prices, and land alienation in Ratanakiri has been a major problem. Despite a 2001 law allowing indigenous communities to obtain collective title to traditional lands, some villages have been left nearly landless. The national government has granted concessions over land traditionally possessed by Ratanakiri's indigenous peoples, and even land "sales" have often involved bribes to officials, coercion, threats, or misinformation. Following the involvement of several international non-governmental organizations (NGOs), land alienation had decreased in frequency as of 2006. In the 2000s, Ratanakiri also received hundreds of Degar (Montagnard) refugees fleeing unrest in neighboring Vietnam; the Cambodian government was criticized for its forcible repatriation of many refugees.

Question: What practices of the Khmer Loeu were seen to be incompatible with communism?
Answer: traditional customs, religion