So, Communism lacked large-scale support in Cambodia before the U.S. began bombing Cambodia?
Not only did they lack large-scale support, the communist movement (i.e. the Khmer Rouge) literally lied about being communists until September 1977 --- over two and a half years into their rule --- referring to themselves before then as "The Organization".
You have to keep in mind that Cambodia was the most backwards part of Indochina and there was little indigenous base for them to draw from. The communist movement was small and largely composed up French-educated Khmer students and Khmer from the more developed Vietnam. Unlike the Soviet and Chinese Communist Parties, the Khmer Rouge were not a mass movement at all, with only 14,000 members at their peak during the third year of their rule (out of a population of millions). Urban Phnom Penh itself had a small urban intelligensia prior to the revolution that was largely bourgeois nationalist, not communist. But the countryside contained the vast majority of the population, feudal peasants who largely supported the French imperialists and then the Sihanouk regime.
Sihanouk managed to lose support of urban Khmer because of the war, which wrecked the urban economy through runaway inflation. This loss of popular support in the capital led to the American-backed Lon Nol coup against him and the establishment of the Khmer Republic (which was violent, nationalistic, and engaged in mass pogroms). It was only when Sihanouk lent his support to the Khmer Rouge in a desperate attempt to take back his throne (lol) that the revolution gained mass support from the royalist peasantry, allowing a victory over the Khmer Republic to be possible.
No war, no Khmer Rouge. The corrupt Sihanouk regime likely limps on until at least the 1990s, despite growing nationalist agitation to take on the Vietnamese and develop the economy. Though of course there may be pockets of Khmer Rouge-controlled jungle as there still is to the present-day IOTL.