Erm this is an interesting question and the only explantation i can see is they are not at regular intervals say as Rugby Union or football after some research i have seen that some years it has been up to 8 years in between and others it has been 2. Why this happens i can only guess there is not enough interest from a country perspective.
There wasn't a world cup in 2004 because the Tri Nations competition was on the rise and the Great Britain and New Zealand teams weren't succeding at intenational level.