There are a number of theories as to why Harry Potter is so short. The most popular theory is that J.K. Rowling purposely made him short so that he would appear more youthful and vulnerable, which would make readers empathize with him more. Another theory is that his height (or lack thereof) is due to the fact that he was neglected and malnourished during his time at the Dursleys', which stunted his growth. Whatever the reason, it's clear that Harry's height (or lack thereof) plays an important role in the story and helps to endear him to readers.