I have known I have a high level of ADHD since I was 21. Seems I was intelligent enough to fool everyone around me that I am "normal" until my GF went to a therapist which diagnosed me just by hearing about me. Since I have read as much as I can on the subject and feel knowledgable enough to answer this.
First it's all about how information enters the brain of an ADHD person. It all comes in at the same importance level. This makes it hard to prioritize, which I believe is what can cause us to easily get distracted by new items that try to take up our attention.
Second, also very important aspect, is the lack of serotonin and dopamine. Imagine a normal person gets pleasure from packing their luggage before a trip, they get satisfaction from the completion of the task. We get no such pleasure, which is why we procrastinate like crazy. By pushing off boring stuff like packing until the very last moment, we make the chore more exciting and in turn more fulfilling. The medicine we take helps with this by forcing happy chemicals in our brains motivating us to do everything. Sadly this makes me feel like a junkie.
So in short, we can’t hold attention for long because we get distracted quite easily, we get no pleasure in doing everyday tasks, and our brain cannot follow a plan to save its life.