To look at why this affects higher frequencies more, just look at this in terms of wavelength.
At 300MHz for example, one wavelength is just 1 meter, or a little over 3 ft., so a short length of feeder can be significant. But at 60Hz powerline frequency, one wavelength works out at over 3000 miles, hence the effects are much smaller.
This is all tied in with the capacitive and inductive effects and the resonant frequency, as given by the formula above (in case anyone hasn't figured out the 6.28, it's 2 times pi). This is why a small capacitance of, let's say, 10pF will have virtually no measurable effect on a 60Hz power line, but can play havoc with UHF television signals.
