Frequencies below 30 MHz have a hard time penetrating the so-called ionosphere - the series of layers of ionized atmosphere that exist at high altitudes.
This is good news for folks like me, amateur radio operators, because it allows us to speak into a transmitter at home, and let the ionosphere bounce the radio waves back to Earth, then again up into the ionosphere, and so on, eventually reaching another radio operator on the opposite side of Earth.
But it's bad news if you're a radio astronomer trying to receive such frequencies from distant objects - the ionosphere simply bounces those waves back into space.
So, apparently, you could raise a receiver above the ionosphere, in a satellite, and have it receive signals from outer space. But there's a catch. At 30 MHz and below, the wavelengths are huge. 27 MHz - the wavelength is 10 meters. 14 MHz - wavelength is 20 meters. 3.5 MHz - 80 meters. And so on, the lower the frequency, the bigger the wavelength.
The size of the antenna needs to be comparable to the wavelength if you want to receive signals from one direction only, so as to be able to tell where the signal is coming from. This also helps with efficiency. A very good antenna needs to be much bigger than the wavelength for it to be directional and efficient.
The problem is, even with the ionosphere, there's a lot of signal "leaking" out of the Earth, through ionosphere, into space; some of it is bounced back down, but some of it penetrates the ionosphere and escapes into space. A satellite in low orbit would be swamped by transmitters from Earth, completely erasing any signals from deep space. You'd need a humongous antenna to ignore the signals from the nearby Earth. This is not feasible.
It would be a lot better if you could just put the Moon in between Earth and your antenna. Even these low frequency transmissions cannot penetrate the Moon, and can't go around the Moon either. So the Moon acts like a screen, casting a shadow on the antenna and eliminating parasite transmissions from Earth.
Now your satellite can receive low frequency transmissions from deep space.
TLDR: Too much noise near Earth at these frequencies, need to hide the antenna behind the Moon.