Will Durant sums it up well:
"The economic causes of Rome's decline have already been stated as prerequisite to the understanding of Diocletioan's reforms; they need only a reminding summary here. The precarious dependence upon provincial grains, the collapse of the slave supply and the latifundia; the deterioration of transport and the perils of trade; the loss of provincial markets to provincial competition; the inability of Italian industry to export the equivalent of Italian imports, and the consequent drain of precious metals to the East; the destructive war between rich and poor; the rising cost of armies, doles, public works, an expanding bureaucracy, and a parasitic court; the depreciation of the currency; the discouragement of ability, and the absorption of investment capital, by confiscatory taxation; the emigration of capital and labor, the strait jacket of serfdom placed upon agriculture, and the caste forced upon industry: all these conspired to sap the material bases of Italian life, until at last the power of Rome was a political ghost surviving its economic death."
Durant is an historian, not an economist, and occasionally his lack of theoretical economic framework hampers his analysis. Generally, though, his conclusions are consistent with the evidence and very convincing.
After the discussion of economic causes of the decline, he addresses the political:
"The political causes of decay were rooted in one fact–that increasing despotism destroyed the citizen's civic sense and dried up statesmanship at its source. Powerless to express his political will except by violence, the Roman lost interest in government and became absorbed in his business, his amusements, his legion or his individual salvation."
He notes Gibbon's conclusion that the Christian religion was a cause of the decline of Rome, but responds, "… the growth of Christianity was more an effect than a cause of Rome's decay." He elaborates, but I'll spare you the details.
The careful reader of this blog will note the long periods between my comments based on Durant (Economic Lesson from Ancient Rome in video, Italian Financial Crisis: A.D. 33, and What's Wrong with Egypt's Economy) The book is full of great information, but it's not exactly a page-turner. (And don't ask me how Gibbon is coming.)
If you're looking for an easier explanation of Roman history, try the History of Ancient Rome from the Teaching Company. I'm part way through the audio course, and the lecture is lively and engaging. (And the Teaching Company offers old-fashioned customer service. I called about a cracked CD, they answered the phone on the second ring, and they promptly sent a replacement, at not charge.)