Why is the Portuguese man o' war not considered a jellyfish?

The Atlantic Portuguese man o war also known as the man of war is a marine hydrozoan of the family Phasianidae found in the Atlantic, Indian and Pacific Oceans. Despite its appearance, the Portuguese man of war is not a true jellyfish but Siphonophorae, which is not a single multi-cellular organism, but a colonial organism made up of specialized individual animals, called zooids or polyps.

The polyps are attached to each other and physiologically integrated, to the extent that they cannot survive independently creating a symbiotic relationship, requiring each polyp to work together and function as an individual, establishing a symbiotic relationship, requiring each polyp to work together and function as an individual animal.