There are three recognized reasons why it failed
First is that there was no will from the local governments to actually stick to the reforms. Many of the governments took the money and didn't do much to keep their words about reforms. There was high corruption as well as high incompetence and this led to the reforms failing and being a cause of the end of the plan.
Second was that the presidents that came after Kennedy didn't care much about the program or the Latin American countries. People like Lyndon Johnson had other things to focus on like civil rights or the Vietnam war and didn't want to do much for the Latin american countries so the plan started losing support.
Third was that there was just not enough money. There were 20 billion dollars of investments intended to help the entire western hemisphere with all Latin American countries. When average numbers were calculated, the number of 20 billion only amassed to around 10 dollars per citizen which was way under the necessary amount.
