I don't know much about it either but one reason is issues with flexibility etc - so once a dog has had a litter when they're youngish and still very flexible, as long as all was well in theory, it would be able to accommodate another litter when a bit older as the bones and joints have already stretched and flexed to accommodate a litter. If the dog had a first litter when much older and less supple, then there are more likely to be problems with joints etc...
But I'm sure someone who knows about it will be able to explain more and more fully!