Why Both are 50-ohm coax, but RG-213 is physically much larger (about 0.405 inch diameter versus about 0.195 inch for RG-58). The bigger center conductor and shield mean less resistive loss per hundred feet at any given frequency, and the thicker dielectric also lets it handle more power. Impedance is the same, so the practical difference is attenuation and power rating.
Watch out The claim that RG-58 handles higher power reverses the relationship: thin cable has a small center conductor and thin insulation, so it loses more signal as heat and breaks down at lower voltages. Shield count is not the defining difference here, and there certainly is a significant difference between the two.
Fatter coax, fewer losses. RG-213 is the thick one, so it wins on both loss and power.