The purpose of a studbook is to show lineage. The development of breeds is based on a closed lineage. Most breeds are developed, other dogs not brought in, move forward. For the AKC and other national stud books, breeds that want to open it are rare… ie developed breeds. I am not sure if there shouldn't be some process for petitioning AKC to do it longer, and in fact not sure how it works (ie can they reopen it every so many years? is there a limit to how many times?)
But in reality, it makes sense to me to make it a 5 yr window. That allows time to add dogs, then to see what that addition DOES to the breed. Let's say we let in new dogs and in 2 generations we have Basenji with green tails. If the stud book has been closed, we can more easily narrow down the green tail gene carriers. In a practical sense, it allows new in, but time to fine tune those new genes to fit what we want (ie the breed standard), and get things stable before allowing in even more. It allows a controllable fix if you let in something disastrous.
When you introduce new genes, especially talking here about native stock with no genetic studies behind them, no generations of observations... you don't really KNOW what will happen. Allowing short term really is a way to bring in new in a controlled manner.