Probably because its a US Mag! So they're a lot more aware of what goes on locally in the states than they are about anything happening outside the states (after all, we all know the world revolves around the USA

).
So, if you're from the states, then you need a few top results in some of their bigger (but still) local events to get nominated (not to exclude US athletes who are at the very top level in the major international events).
But, if you're from the rest of the planet, you need to be winning the biggest/most major international events, and primarily those major events that occur in, or have support from a large number of US athletes (eg. BOP, ISA). On top of that you've probably had to be up there near the top of the world stage for a couple of years at least, in order for the Americans (who form the majority of the population nominating and voting on the awards) to notice you as an athlete.
So basically, the Aussies (and other paddlers from the world outside the states) who have managed to get nominated, have been getting good results on an international stage for a number of years and probably deserved to make the list a year or possibly two before their name first appeared.
For the other top Aussies who didn't make the list, its probably more due to them not doing enough races/events in the USA to get recognised by the US Sup population, or they haven't taken down the big US names at a major event. Nothing to do with them not having the ability or credentials to be worthy of a place on the list.
All the more reason to congratulate the Aussies who did make the cut, and some of the other elite crew can get up there in the future.