The Irish consumed potatoes because enough to feed a family could be grown on a relatively small plot, they could be left in the ground, dug up through the winter, making them difficult for the English to requisition (steal), and combined with dairy, potatoes had all the essential amino acids.

The reason the blight struck so hard is that the potatoes being grown were all essentially clones of each other. Planting a new bed was a matter of cutting the eyes out of the potatoes you already have and planting those pieces. FWIW, I made it a bit of a quest to eat one of those potatoes as I thought it might be amazing. Turns out there aren't any, but someone in England managed to recreate the line. It was a bit of a let-down. The Irish called this strain of potato "the Lumper", and that name plus the unexciting taste tells me the attraction was that it grew large and so provided plenty - until the blight, of course.

So... clones. Microorganisms evolve quickly. Larger plants and animals do not. You can visualize that larger organism's immune system as a set of 'locks' the microorganism is trying to get past. There's some thought that sexual reproduction (in the sense of an offspring's genetics being a mix of the two parents) was a solution to the mismatch in evolutionary speed.