Understanding the Maximum Speed that can be Transmitted over a Cable
The maximum frequency is mostly related to the frequency-dependent loss characteristics of the cable. Eventually you get to a frequency where you simply don't get enough signal at the other end to use.


Resistive losses in the conductors (including skin effect)
Dielectric losses in the insulating materials
Radiation losses if the cable is not fully shielded


All of these tend to increase with frequency.

This is why we generally switch to other technologies at very high frequencies: waveguides for microwave radio equipment and optical fibers for high-speed data.