The effect is apparently small enough that they had to use Fourier analysis over many minutes to see it in the noise.

There are many millions of devices connected that draw power, and increasingly so, supply power.

>> explain why this was apparently so difficult to find out?

Likely because the source of the noise was a class of energy providers that themselves react to fluctuation in the grid. Electricity may travel at the speed of light but all the little switches and transistors that connect/disconnect suppliers do not. It would have been noise within noise.

If it were not a substantial research effort spanning years, it would not get funded. This motivates academic researchers to ensure that every topic is explored thoroughly and at great length.