Realpolitik. The People's Republic of Kampuchea didn't fall so much as adapt.

Pol Pot's People's Kampuchea was Chinese-aligned and horrendously unstable. Vietnam was hostile to China and didn't want a chaotic enemy-aligned regime on it's Western border. Thus, they invaded to overthrow Pol Pot's regime and installed a regime sympathetic to them, the People's Republic of Kampuchea under Hun Sen.

Basically throughout the 80s Vietnam occupied Cambodia, propping up the PRK- which honestly was an improvement over Pol Pot's genocidal madness. Towards the end of the 80s, Vietnam withdrew from Cambodia under international pressure (and given the drying up of Soviet funding given that the USSR had it's own problems) and Hun Sen's government started engaging in the same sort of inching towards economic liberalisation that Vietnam itself was beginning to engage in.

As part of this process, a royalist party under Prince Norodom Sihanouk began to contest elections in the early 90s which led to huge tensions between the Communists and Royalists. It helped that Sihanouk genuinely had Socialist credentials, and had been involved in Cambodia's initial struggle for independence from France, and in Cambodian politics after independence (as a politician not a royal), and it was his overthrow that had led to the takeover of Cambodia by Pol Pot.

Sihanouk cut a deal with Hun Sen which satisfied both parties with Hun Sen participating in a coalition as head of government and Sihanouk becoming head of state under a constitutional monarchy.

End result, Hun Sen is still Prime Minister, and the PRK apparatus essentially transitioned into the modern Cambodian state apparatus.