because English is weird! "endemic" is close in meaning to "native". Also, you would use different articles to say them. Jack is a native of Australia. native = noun Jack is native to Australia. native = adjective Smallpox is endemic to Australia. endemic = adjective "Smallpox is an endemic of Australia" is not correct. Endemic is a noun sometimes, but it is not used this way. Also correct: Smallpox is native to Australia. native = adjective Smallpox is a native disease of Australia. native = noun Smallpox is a disease endemic to Australia. endemic = adjective Hope this helps you.