- Most land were controlled by large land owners. Most peasants had very small farm land, which couldn't feed the people if normal crops were planted so they had to plant the higher yield potato to have enough food. When the disease wiped out the potato crop, most people went without any food.
- The land controlled by the large land owners were planted with cash crops for export. They were unwilling to stop the export to help the locals.
- The ruling class consisted of large land owners and British transplants in the Northern Ireland after the conquering of Ireland by Britain could care less about the shortage of food. They actively hid the problem from the central government in Britain. The governor/duke/whoever tried to sweep the problem under the rug to avoid appearing as incompetent.
- When the central government in Britain learned of the famine, they acted too late and too little, unwilling to spend money to deal with the emergency. Britain at the time looked down on the Irish people in general.
This led to great animosity of the Irish people to Britain, driving the subsequent independent movements.
Fun fact, about 10% of the U.S. population are of Irish descent, due to massive immigration from Ireland in the following years after the famine.
Edit: Just looked up the Irish population in U.S., about 11%. https://uscanadainfo.com/irish-ancestry-in-america/