There were three main causes of the Americanization of the war.
First, the French were unable to win the war on their own. They were unable to win even with American financial help. Once they left in 1954, the US had to step in if it wanted to keep South Vietnam from becoming communist.
Second, the South Vietnamese government was weak and unpopular. The South Vietnamese were not particularly dedicated to trying to win the war. Therefore, they were doing badly against the Viet Cong in the years up to 1964.
Finally, the US was very concerned about the spread of communism and about the image of the US. It felt that it had to keep South Vietnam from "falling" so as to contain communism and to affirm that the US would stand by its allies. This made the US willing to completely take over the war if that was the only alternative to having the communists win.