Here's why:

* Transverse waves have oscillations perpendicular to the direction the wave travels. Imagine a rope tied to a wall. If you shake the rope up and down, the wave travels along the rope, but the rope itself moves up and down (perpendicular to the wave's direction).

* Compressional waves have oscillations parallel to the direction the wave travels. Think of a spring. If you push one end of the spring, the compressions and expansions travel along the spring, with the spring itself moving back and forth in the same direction as the wave.

In an electromagnetic wave:

* Electric and magnetic fields oscillate perpendicular to each other and to the direction the wave travels.

* This perpendicular motion is what makes electromagnetic waves transverse.

Let me know if you'd like a more detailed explanation of electromagnetic waves!