I need some help on an assignment responding to the following question: Why does Catholic doctrine not consider Sacred Scripture alone as the Word...

There are actually two types of issue here, one which is common to all Christian denominations and one uniquely Roman Catholic.

First, we have the notion in all denominations that God wrote  "two books", the "Liber" (Bible) and the "Liber Mundi" (book of the world). As the world is as much God's creation as the Bible, one can read evidence of God's design in the world just as much as one can read it in his other book. According to St. Augustine's De doctrina Chistiana, the Liber Mundi signifies directly but the Liber only indirectly (because it uses forms of symbolic representation). 

Next, the term "Revelation", which some people use loosely to refer to the Bible, in most Christian theology refers to God's revelation of himself in Jesus. The New Testament is a book about this revelation, not the revelation itself. 

Historically, the Church developed around this revelation of God in Jesus. The first written texts we have in the Christian tradition are the letters of St. Paul, who never met Jesus in the flesh. Rather than relying on written texts, early Christian belief was formed around orally transmitted stories about Jesus that were eventually written down some fifty or more years after his death; our earliest papyrus fragments of the New Testament date to the beginning of the second century. 

As to which of the works about Jesus were eventually chosen to become part of the canon of Christian writings, that was a matter of vehement debate. Irenaeus, in the late second century, argues for our current four gospels, but he is clearly arguing a point that is under debate rather than established. 

What this means is that the Church, as the guardian of Revelation, selected which books were to become the Bible; the Church is in effect prior to the Bible and determined its composition and interpretation. Also important is that there are many things, such as the details of liturgy and points of theology (such as the nature of the Trinity) that the Bible does not discuss; for the Orthodox and Roman Catholics, the seven ecumenical councils supply some of this missing material. For Catholics, the Church magesterium adds even more to the body of authoritative teaching.

The sola scriptura doctrine which arose in the Reformation rebelled against this primarily for two reasons. First, it thought that the church had become corrupt and departed from the teachings of historical Christianity. Second, many reformers believed that Roman Catholic doctrine gave excessive power to the clergy and the institution of the church. The sola scriptura doctrine essentially empowers the laity.