In short, it was the fault of culture. In England there existed the right of primogeniture which states the oldest son inherits the farm. In Ireland it was split equally between the sons, so quite soon a 100 acre farm became perhaps twenty individual five acre farms. Couple this with overdependence on a single crop and that sets the necessary conditions for a famine. The single crop was potatoes because they are remarkably nutritious, very cheap, and one can grow a huge amount on a small plot of land. So you have the majority of the country on these tiny sub-divided farms that they didn't own surviving off of one crop. In comes the potato blight and that coupled with horrendous mis-management by everybody leads to the slashing of population and massive emmigration such as it was. Hope this answers the question somewhat

I suspect you mean the economic reasons and not the biology.

The land owners continued to export valuable food out of the country at the expense of the poor, non-land owning populace.

So in short, there was enough food but it was sold for profit instead of being used to prevent famine.