The reason is that building up larger and larger elements is a lot like building a tall tower of blocks. Each block that's added makes the tower more and more unstable. And in the case of fusion, each successive, larger element puts out less and less energy. Iron is the end of the line, because fusing iron *requires* energy rather than releases it! It takes one month to fuse Mg into Si, but only *one day* to fuse Si into Fe.