The unshielded conductors radiate an electric noise field that couples capacitively with adjacent wiring. Stray capacitance at A & C cause ground currents to flow creating a magnetic noise field that couples inductively with adjacent wiring.
Note! Not all drives allow the use of a ferrite sleeve around power conductors. Refer to your manuafacturer’s manual for specific applications.
Magnetically soft materials, however, tend to be more expensive. The motor manufacturer must find that proper blend of just enough magnetically soft material to do the work required without putting too big a dent in the customer wallet.
Think of the atoms of magnetic material as an unruly herd of cattle. Running electric current through the material will polarize these atoms, creating the magnetic field. But as I mentioned, this is an unruly herd, so it takes time for the current to bring all those atoms into formation.