Why is macau a special administrative region?

Because it was a Portuguese colony for almost 450 years and was only returned to China in 1999. It was under Portuguese control since the mid 1500s and its colonial government followed Portuguese law and China ceded it to Portugal. Macau was an important center of trade with Europe in Asia from its founding until the British transformed Hong Kong into an international trading colony in the 1850s. Afterwards it declined and Portugal decided to hand Macau back to China in the 1960s but China refused. As Britain and China were debating over Hong Kong's future, Portugual agreed to handover Macau under the "one country, two systems" agreement that allowed Macau to function as its own nation with only Chinese interference in military and some government issues.