Because marriages and children are a huge change in an already established status quo (unless it's the Fantastic Four). This limits the author's work, it takes away the focus from superhero adventures, it adds the need to pay attention to family life, which can annoy those who are not interested in it. There can be many reasons, in fact.
By the way, this is a much more serious problem for writers. In terms of readers, so far it turns out that most are not particularly against families and children. But for many writers it's really difficult.