Here's why:

* Sound waves are longitudinal waves: This means the particles in the medium (like air) oscillate back and forth in the same direction as the wave travels.

* Compressions and rarefactions are parts of the same wave: A compression is a region of high pressure where the particles are close together. A rarefaction is a region of low pressure where the particles are spread out. These two regions are simply different phases of the same wave.

* Speed is determined by the medium: The speed of sound in a medium is determined by factors like the medium's density, temperature, and elasticity. The compression and rarefaction parts of the wave travel at the same speed because they are both disturbances in the same medium.

Think of it like this: Imagine a long line of people holding hands. If you push the first person, the pressure will travel down the line, causing people to bunch together (compression). At the same time, the space between people will become larger behind the initial push (rarefaction). Both the compression and rarefaction travel at the same speed along the line.