I think it is because they are grown in water, more or less,
so if a few seeds are infected (and they easily could be, as they don't add things like copper sulphate to seeds that will be eaten), the infection is spread over a large number of sprouts. As many sprouts aren't cooked, the germs aren't killed by heat. According to the Food Safety Network's email digest, farmers are supposed to use a seed disinfection procedure immediately before starting the sprouting process, and they suspect that this wasn't done properly. Farmers are also supposed to test the water used regularly.
My mother was very suspicious of germs on all salad-type food, and she taught me to soak lettuce, sprouts etc in salt water before using them, to kill the bugs. At one point, I was a business analyst in the food industry. The horror tales I heard in my work (as well as a bout of food poisoning from a salad in a top restaurant) prompted me to further excesses of caution, and I soaked my lettuce in a mild solution of potassium permanganate to ensure bug death. I washed the lettuce EXTREMELY well after this, but once my friends started noticing what I was doing, they stopped eating salad in my house, and I was forced to revert to the salt water.
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