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Wavelength decides if reflections matter
Wavelength and frequency
Speed equals wavelength times frequency, and on a given line the speed is fixed, so raising the frequency shortens the wavelength. When the wavelength is long, a reflection that comes back looks almost identical to the original wave and barely changes its shape. Once the wave is shorter than the line, the reflection interferes with the incoming wave.
Standing waves
That interference makes a standing wave pattern: at some points the two waves cancel and the voltage is very small, and at others they add together. If the reflections are bad enough, the peak voltages can reach up to twice the input voltage, and a line not rated for that burns out.
Power lines versus radio
Household AC at 50 or 60 hertz has a wavelength of about 5 to 6,000 kilometers, far longer than any wire carrying power from the station to a home. At 10 megahertz, a radio signal has a wavelength of about 30 meters. A radio transmission line more than two kilometers long is many times that, so the reflections on it were significant.
Why can standing waves double the voltage?
At a point where the forward and reflected waves rise and fall together, their voltages add. With a full reflection the two have equal size, so the sum reaches twice the incoming peak. A quarter wavelength away they move in opposite directions and cancel to nothing. These peaks and nulls stay in fixed places along the line instead of traveling, and the ratio of the largest voltage to the smallest is called the voltage standing wave ratio.
Why is a power line's wavelength so long?
Wavelength equals the speed of the signal divided by its frequency, and a signal in a wire travels at a large fraction of the speed of light, roughly 300,000 kilometers per second. Dividing that by 60 hertz gives about 5,000 kilometers, and by 50 hertz about 6,000. Power lines are rarely more than a few hundred kilometers long, a small fraction of one wavelength. On the longest high-voltage lines, the Ferranti effect raises the voltage at a lightly loaded far end above the voltage at the sending end.
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