I believe that Rugby Challenge has had a longer development time than Rugby World Cup and a larger budget, commitment of resources. Mario from Sidhe has stated that the budget for Rugby Challenge is significantly larger than what they've had to work with before via HES.
I believe that given the fact that HB Studios did not secure a producer/publisher until fairly recently in the development cycle, that it's almost certain that the budget/commitment of resources was substantially lower than what they would've liked to have had to develop this game.
HB Studios are a studio which develops other games (often several projects at once) for companies like EA. I'm sure these other projects which would've already had funding and a producer/publisher wanting progress, would've commanded a reasonable amount more time or personnel resources.
All of this is not to say that HB have had a hard run and Sidhe has had it easier. Sidhe would've had a variety of other difficulties in their development cycle, not least of which would've been largely building up their project from scratch.
Given the difficulties on HB's side, I'm sure developing on the existing platform of Rugby '08 was the only logistically feasible step.
This is complete speculation (but at least I state that in my posts) and I may be well out, but I'd be guessing myself that the budget difference could've been something along the lines of $10million to $2million.