Chemistry and geography, and it is a genuinely good story. Ireland's damp climate suits soft wheat, which is low in the protein that builds the gluten structure a yeast bread depends on. When bicarbonate of soda became widely available in the nineteenth century, Irish bakers finally had a leavener that needed neither strong gluten nor time: soda plus the acid in buttermilk releases carbon dioxide the instant the two meet, so the loaf rises in the oven rather than on the counter.
The cross slashed across the top is practical before it is symbolic. It opens up a dense round loaf so heat reaches the middle, and it makes the bread easy to tear into quarters. Tradition has attached its own meanings, from blessing the loaf to letting the fairies out. And when it is studded with raisins, as ours is, Irish bakers will often call it spotted dog rather than soda bread at all.
Practically, that is why a whole loaf, from flour to sliced and buttered, fits inside a 60-minute workshop. Nothing has to prove. The same trick is why the cake donuts on this page are chemically leavened too.