Answer:
Because their doctrine was that of the cold war.
Step-by-step explanation:
The Truman doctrine was precisely to prevent Soviet influence from expanding, and for this reason, the Truman government supported any government that was allied with the U.S. instead of the Soviet Union, no matter how repressive said government might have been.}
Eisenhower did the same. Hi came up with the "domino theory", which stated that if a country became communist (for example Vietnam), its neighbors would follow soon (Laos would become communist, then Thailand, then Cambodia, and so on). This is why Eisenhower launched the American military invasion of Vietnam in first place, starting the Vietnam War.