Why did the “Vietnamization” of the war first bring success and then end with the defeat of the Saigon regime?

The “Vietnamization” of the war was successful at first, because the Americans contributed to it:
1) They invaded Laos and Cambodia and destroyed the guerrilla base on the Ho Chi Minh trail
2) Under the leadership of American instructors, a combat-ready army of South Vietnam was created, onto which the main burden of warfare was shifted.
But, the fact is that this army could be combat-ready only with constant military-technical and financial support from the United States, And after the resignation of President Nixon (1974), the United States sharply reduced its assistance to the allied regimes in Indochina What could not be said about the USSR, China and the DRV, which continued to support the communists. in Laos, Cambodia and South Vietnam. And in the spring of 1975, the communists in these countries went on the offensive. In March 1975, the Southern Army of Vietnam was defeated, on April 30 of the same year, the communists entered Saigon, the capital of South Vietnam, and renamed it Ho Chi Minh City, in honor of the President of the DRV who died in 1969.