Normal matter loses energy through self-interactions (collisions), which is radiated away. Because the emission is isotropic, angular momentum is conserved even though the total energy isn't. This tends to make the normal matter collapse into a disk, which must rotate faster than the original halo to conserve angular momentum. Dark matter, which hasn't dissipated any energy (gravity is conservative), can move around more, never decaying into the orbits that the normal matter does.