Other than the extra 2/3 million per year he'll likely get after a 4/5 year deal....no reason at all!
It's still a risk on his part if he decides to go with a shorter term contract. Just as a longer term contract is a risk for the team.
I don't think anyone can argue strongly that either a short or long term contract is what Brodie would obviously go for, because there are arguments for both sides of it from a player's point of view. From the team's point of view, however, I really do think an 8 year contract is ideal in this case, to improve likely cap efficiency 3+ years down the road when the team might actually be competitive.