Because MAF make it too difficult!! You don't need permits to keep exotic reptiles vs native ones, native fish aren't very widely available (or half as diverse and colourful as tropicals), not too sure about hte legality of keeping native birds, but I know you're not allowed to breed them to release. And as for tuatara...
Thats a pretty stink attitude really, how many exotic pets are out of control? I haven't heard of beardies roaming free and eating native insects, or oscars taking over native streams and eating the native fish. Yes the rainbow lori's are a problem (and rosellas) but the fact is that many animals (cats, dogs, rabbits, guinea pigs, rats, mice, goldfish etc) are all perfectly legal to keep and breed, yet they pose a much higher threat to the native flora and fauna than a lot of "exotics"...