Why Brass is a nonmagnetic but electrically conductive metal, so its relative permeability is essentially 1 and it does nothing to concentrate the magnetic field. Instead, the coil's changing field induces eddy currents in the slug, and those currents create an opposing field that cancels part of the coil's flux, lowering the effective inductance. That is why brass tuning slugs are common in VHF and UHF coils, where you want to trim inductance downward.
Watch out Ferrite is the opposite: its high permeability concentrates flux and raises inductance, which is what most slug-tuned coils use at HF. Ceramic is an insulator with permeability near that of air, so it acts as a form and barely changes inductance at all.
Magnetic core (ferrite/powdered iron) raises L; conductive nonmagnetic core (brass) lowers L via eddy currents.