The correct answer to this open question is the following.
Although there are no options attached we can say the following.
The Eastern province of the Roman Empire did not decline as drastically. Based on what I've read, I think the Western province fell because of the following reasons.
There was just non one reason for the fall of the Western Roman Empire. I was more the consequence of a series of events.
One of the most important was the invasions of Barbaric and German tribes. In 476, the German leader and his troops invaded Rome and capture the city. Rome had fallen.
Another factor was that the western portion of the Roman Empire was so vast, that the Emperor lost control of those faraway territories. We have also to consider a series of economic problems at that time. Constant fights caused the overspending of resources due to the war.
Under these circumstances, political and economic corruption spread through Roman politics and this erode Rome from the inside.
