Why don't all the electrons in an atom fall to the lowest energy level?
It violates the Pauli Exclusion Principle, as the ground state is the most stable, it's natural to wonder why an atom can't contain all electrons in the 1s subshell. Briefly, the electrons would repel each other in the same orbital.
Pauli Exclusion Principle
Look at "applied to atoms". More specifically, no two electrons may have identical quantum numbers