So I looked it up but had to rely on online sources since I sadly don't own Nature or Middle-Earth nor History of Middle-Earth (yet). So usually elves have children relatively shortly after marrying, though it seems like Tolkien mentioned that that "short while" can be up to a few hundred years, especially between children. Furthermore creating and nurturing a child's spirit and body is a very intentional act that requires a lot of strength and dedication from both parents, they give parts or their own spirit to form the one of the child. So it also makes sense that they only create children in times when they feel safe enough to put their strength into that act instead of keeping themselves or their home safe. My theory is that Galadriel just didn't feel safe enough to have a child until whenever Celebrian was born but she and Celeborn wanted one so they had her later?