I have just read this excellent book about mathematics, and I can answer your question quite simply: The Egyptians and the Chinese knew this law 1000 years or so before Pythagoras, so they are the discoverers of this law. Pythagoras, however, is almost always credited as the contributor. (Don't you find it cute that the Archimedes' Screw is called so called even though he didn't invent it? What is with the ancient Greeks?) Pythagoras has a fascination for proving numbers, and he is credited the most because it was he who PROVED that it goes for ALL right-angled triangles. He is a renowned mathematician, and proved this theorem because he believed in proving something first instead of giving a sweeping statement. His other works are 'perfect numbers like 6 and 28'.