Here's why:

* Magnetic Properties: Iron oxide is a magnetic material, which is essential for recording and playback. The magnetic particles align themselves according to the audio signal, creating a magnetic pattern on the tape.

* Durability: Iron oxide is relatively durable and resistant to wear and tear, making it suitable for repeated use in tape recorders.

* Cost-effectiveness: Iron oxide is relatively inexpensive to produce, making it a practical choice for mass-produced tapes.

While other materials like chromium dioxide (CrO₂) and cobalt-doped iron oxide (Co-Fe₂O₃) were also used in some higher-quality tapes, iron oxide remains the most common and historically significant material for magnetic tape coatings.