Moving one patient onshore for treatment is easy. Moving dozens of intubated patients all at once because a particularly nasty strain of the virus has swept through not so much, especially given WA is the only feasible destination and has seemingly minimal ability to treat or manage cases.
Also Christmas Island is a long way away. Like a really long way, further from Perth than Auckland is from Sydney. It’s just not feasible logistically to be ferrying people and resources back and forth over such a large distance.