This is somewhat of a politically loaded question. The first question would be what was rate development during the reign of Haile Selassie. It's very much true that Haile Selassie did much to modernize the country especially in the 1930's and 1940's, but for the average Ethiopian peasant not much changed under Haile Selassie.

The biggest problem was that Haile Selassie never really tackled the issue of the nobility and aristocracy. The nobility and aristocracy dominated agricultural production, which is the backbone of the Ethiopian economy. It was basically a fuedal system that really had no place in a modern economy. In 1973 there was a massive famine in Wollo that killed up to 300,000. There is some debate as to what exactly caused the famine, whether it was simply a decline in food production or the inability of peasants to purchase (obtain food). A contributing factor has been identified as a lack of infrastructure (roads) to get food to areas experiencing famine. Nevertheless, the fuedal system in place contributed to the famine in 1973, that and the Haile Selassie's lack of response lead to protests leading to the overthrow of the monarchy.