This is a more technical question, showing the user is looking for a deeper understanding of the physics.

In a transmission line, the most common mode of propagation is the Transverse Electromagnetic (TEM) mode, where both the electric and magnetic fields are perpendicular to the direction of wave travel. This is the simplest and most efficient form of energy transfer for the frequencies our equipment handles.

In a waveguide, TEM mode cannot exist. Instead, the waves propagate in more complex patterns known as Transverse Electric (TE) or Transverse Magnetic (TM) modes. This is due to the single conductor (the hollow tube) or dielectric nature of the waveguide structure, which forces the electric or magnetic fields to have a component in the direction of wave travel.