The simplest answer to this was that there wasn't anything else to eat, or at the very least, not nearly enough of it.

Ireland had come to depend massively on the potato as a food source since its introduction.

It had taken over from other foods - cereals and meat animals on grazing lands - because it really will grow in places that nothing else edible will. You can plant it in a bog, on a mountaintop, in terraces on slopes where nothing but a goat will otherwise venture, in barrels, in buckets, on a roof. And so on. In the tight confines of Ireland, where a great deal of land is unsuitable for any other crop, it made a vast difference. And potatoes keep really well, such that winter storage was genuinely a matter of heaping them up in the dark. Further, until the coming of the blight - which was, astonishingly, probably from one single tuber imported from Mexico via the US in the late 1830s or early 1840s - it was a very disease resistant plant, most likely because there was nothing in Europe that was inclined to attack it. So with all of this, other crops, where they were grown, were being sold (via the English landlords, for the most part) and shipped out of the country. The Irish lived on potatoes and milk, and the odd bit of fish.

So for 200 years or so, people got seed potatoes, planted them, those produced lots more potatoes, of which some were sown, and the cycle repeated. This fed families from land unsuitable for anything else. And so population increased. For, as noted, 200 years. So by the time the blight did arrive, the island of Ireland had a population of around 8 million people. In the mid 1600s, before the potato, that was around 3 million. So when the potato crop failed, there simply was not enough food to sustain the population.

There's plenty written about the English shipping out food when the Irish were starving, and that certainly happened. But the reality is that even if all the food produced on the island has been kept there, people would still have starved without the potato crop. And if all the ships available had been pressed into service bringing food to Ireland - were that possible - it still wouldn't have been enough. The population was just too big for the island to support without the miracle food of the potato.

Even today, with 21st century farming techniques, globalised supply chains, and a vastly higher urban population, there are only about 6.5 million people on the island.

[I have had serious deja vu writing this - apologies if it's similar to something I wrote before!]

References

Cormac Ó Gráda, The Great Irish Famine, 1995.

Cormac Ó Gráda,, Black '47 and Beyond The Great Irish Famine in History, Economy, and Memory, 2000

Redcliffe N. Salaman, William Glynn Burton, The History and Social Influence of the Potato, 1985.

S B Goodwin, B A Cohen, and W E Fry, 'Panglobal distribution of a single clonal lineage of the Irish potato famine fungus', Proceedings of the National Academy of Sciences of the United States of America, November 1994, Volume 91, No. 24.