Answer:
Because it was the first time Rome had been conquered in 800 years!
Another reason was the fact that Alaric held the Roman title of "master of soldiers" making him a valued member of the Roman Empire and most of the vasgoth soldiers were trained by the Roman Army..
Since most Roman citizens by this time had lost the taste for fighting... often Roman youth would cut off their index finger of their right hand making them unfit for military service.
In addition, we find that Alaric first gave the roman senators young men that were old enough for fighting but still too young to shave... as slaves.
This young men were to on a set date kill their masters and then go to the gate and kill the guards... open the doors and let the Goth army walk in...