Here's why:

* Frequency refers to the number of complete wave cycles that occur in a given time interval. It's a measure of how often a wave repeats itself.

* A single wave pulse is a single disturbance that travels through a medium. It doesn't have a repeating pattern, so it doesn't have a defined frequency.

Other properties that can describe a single wave pulse:

* Amplitude: The maximum displacement of the medium from its rest position.

* Wavelength: The distance between two corresponding points on a wave (e.g., two crests or two troughs).

* Speed: The rate at which the wave pulse travels.

Let me know if you'd like a more detailed explanation of any of these concepts!