why does the nucleus occupy a miniscule fraction of an atom’s volume, but accounts for nearly all of its mass?

Answer 1

Because electrons are nearly massless when compared to protons and neutrons. An electron weighs 1/1836th of a proton. So, even though the electron cloud accounts for a large volume of an atom, the majority of the mass is concentrated in the nucleus, with the protons and neutrons.

Answer 2

because protons and neutrons are about 1000 times heavier than the electrons and they only stay in the center while the total volume of the atom is computed based on the radius of the outer electrons orbit so most of an atom is empty space.

Answer 3

Because electrons take up a lot of space. Nucleons are bound together by the residual strong force, but the electrons insist on each maintaining its own orbital space.