Motherhood and fatherhood are both very important and neither should be devalued. With that said, not every mother or father, or pair of mother and father, is ready to raise children. Some are ready at a younger age, some at an older age. Some have other things to take care of and aren't really well-equipped to put their children first.
From what I've seen, families with both a mother and a father are usually better off when it comes to raising children. This isn't to say that single parents cannot handle it, because I have seen some who make excellent parents, just as I have seen parents in nuclear families who weren't so successful. However, naturally, families with a mother and father are usually more successful at raising children.