Why is the continuation of the human atomic structure an insufficient explanation for continued personal identity of an individual?
If subject "a" remains subject "a" on an atomic level surely that constitutes the continuation of that subject. Arguably the atoms change over time, but not all at once. If say one atom changes on Monday, and then next on Tuesday, the very fact that an atom from Monday remains on Tuesday (even if it was the new atom on Monday) allows for the continuation of that subject.
This simplistic example shows how on a basic level something of the person remains prior to the present moment.