As it turns out, the answer to 'Why' is actually quite practical; as all of my New Testament sources - Johnson, Robert & Feuillet, and Harrington - unanimously agree: "he writes to the Romans, preparing the way for them to become his new base of operations in the West." (Johnson) "As Apostle to the Gentiles he was more anxious than ever to establish contact with the Roman church for, in view of that apostolate, its position as the church of the empire's capital was of paramount importance. ... [He] saw clearly that the roads which led from Rome to all parts of the orbis Romanus could become so many roads of missionary expansion." (Harrington) All roads lead to (and from) Rome.
But an obvious question would be, why is the epistle so long and rich? Johnson makes the point that Romans is Paul's recommendation letter for himself. He's explaining his Gospel. He's telling the Romans, "I know you have your own foundations, but this is what I'm all about, and why we should work for God together." Remember Paul didn't establish the Roman church, and he wasn't writing at any particular crisis in the community like he did in so many other epistles. He's outlining the themes of his mission so that the Romans can understand his intentions of their place in it.
That does not mean Romans is a systematic exposition of Paul's theology. He leaves out a lot of his important theology that he wrote to other churches. "... on this point the Reformers of the sixteenth century exaggerated its value and character." (Robert & Feuillet) Rather, the Roman epistle is a concentrated narrative of the theme of God's salvation - first foreshadowed, presented, and preached to the Jews and then to the Gentiles as well. It is a compelling history of salvation for the whole world. But I don't think it should be applied as a systematic theological dogma of salvation for the individual person.
