Because by and large Cyberpunk is a genre which deals with moral dissonance to our world and the consequences which spring from allowing totalitarian governments and corporate interests to overtake our humanity, as well as the journey characters take to find their humanity in such a society despite these things.
Transhumanism is about shedding, at the very least, our physical humanity by bodily augmentation, but often extends to mental metamorphosis via uploading of consciousness, what it really means to be alive, etc.
Cyberpunk often borrows elements of transhumanist stories in order to further show the dissonance between the characters that have bodily modifications, detachable limbs, robotic organs, sometimes even entirely computerized brains, and their corporate, polluted, 'false' world. Contrasted with normal humans, who are physically and mentally inferior, but live in a 'true' world, complete with what one could call a 'soul' or 'spirit'.
The seminal works of both genres, such as Blade Runner and Ghost in the Shell all deal with these themes heavily, among many others. Watch them.