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.