Thanks for the question! (B) is wrong because it doesn’t strengthen the argument as well as (E) does. First of all, (B) is talking about nutritious food that isn’t flavorful, and the argument is about getting people to eat nutritious foods that are flavorful. Also, (B) is saying that people wouldn’t eat foods that are nutritious but not flavorful. But we want people to eat foods that are nutritious, and (B) just talks about people not eating those foods. And of course, on the LSAT, we have to pick the best answer; (E) strengthens the argument more strongly than (B).
Hope this helps! Feel free to ask any other questions that you might have.