Because both "hurt" and "pain" have the meaning of "ache" when I looked up them in a dictionary.
Please help me.
The quick answer is - as so often - because the collocations don't work. The words do have overlapping meanings, but they don't fit in the same contexts. The pain you have in your stomach is often a 'stomach ache', but when there's a specific cause of the pain it can be, for example, 'a stomach cramp'.