That would be more difficult, expensive, and unreliable. The light is powered by the rotation of the wheel so it already has a reliable braking signal.
That sounds much simpler, cheaper, and reliable than "detect changes in the frequency of the wheel rotation".
Also, it works like a car's brake lights, which drivers are accustomed to, rather than "start pulsing when you decelerate", which is when you stop pedaling (unless going downhill).