perhaps because it has it's own tectonic plate and thus is no longer just an island?
Using this logic, Eurasia could be considered an island as well. You have to draw the line somewhere. They're all empiric definitions anyway.
In the english speaking world we refer to Australia as a continent, but I just saw a kid's encyclopedia from France which refers to Australia as an island.
There oughtta be 7 continents; without Aust there'd only be 6 :(
In Germany Australia technically belongs to Oceania which includes Australia, New Zealand and all those tiny islands. Could be the same in France.
It's really no big deal, but in Australia as kids we're taught we live on a continent that is an island and a country :)