Some people use distractibility and impaired sustained attention interchangeably, but they are different. Some people have quite strong abilities at being able to sustain attention, but they may also be exquisitely sensitive to distraction.
In this study, we get a chance to see why distractions make it hard to learn. The areas of the brain (prefrontal) that are necessary to resist distractions are exactly the same areas that are needed keeping information in mind. It looks like a direct competition.
Other interesting details - we are more sensitive to distraction at the limits of our memory (the more challenging the task) and when the distracting sounds are fluctuating, rather than constant like a hum.
Auditory Distraction pdf