Why An inductor's inductance depends on how easily the core material supports a magnetic field, which is its permeability (symbol mu). Inductance scales directly with permeability: L = mu times N squared times A divided by l, so swapping an air core for a ferrite or powdered-iron core with permeability hundreds or thousands of times higher multiplies the inductance by that same factor. Permeability is the magnetic analog of permittivity in capacitors.
Watch out Permittivity is the tempting look-alike, but that is the dielectric property that sets a capacitor's capacitance, not an inductor's inductance.
Permeability = magnetic (inductors); permittivity = electric (capacitors). Both start with 'perm', pick the one with 'mea' for magnetic.