So the exact reason behind why isn’t 100% understood, in fact it took ages to figure out which foods were more likely linked to infant botulism (like honey) and is just a very easy thing to eliminate from infants diets to lower the risk. Infants aren’t born with a developed GI flora of good bacteria which help defend itself, instead their intestines are basically ready to develop their flora by being exposed to good bacteria that can colonize and develop a defense network, thus when the spores are introduced to an infants GI tract, they’re far more likely to colonize and take over in an infant and begin producing the actual botulism toxin that causes infant botulism. An adult’s flora is well developed and the spores don’t have a chance to colonize and begin producing this toxin in the intestines before being destroyed.

However, adults can still get botulism but this is because they eat food that doesn’t just have the spores there, it’s because they’re eating food that has already had the botulism colonize in it, thus it’s already a colonized/grown and heavily present with the actual botulism toxin that the bacteria colonies produce. Nearly anyone that ingests food already colonized with botulism bacteria is going to end up with foodborne-botulism regardless of age of intestinal flora development. An infant is just more susceptible to infant botulism because their intestines are susceptible to allowing the spores to begin colonizing, producing botulism bacteria, and having the botulism toxin produced inside of them.