If you point your radio telescope at the sky there are all kinds of signals. Some come from the galaxy itself. Some come from our atmosphere. If you made a diagram of this unavoidable noise, you would notice that it's really noisy at low frequencies because of the galaxy. It also becomes noisy at higher frequencies due to atmospheric noise.
Nature provides an even nicer way to further refine our frequency range.
Another molecule in space, the hydroxyl, or OH, emits at about 1.64 GHz. This would be a good, and nicely limited, range of frequencies to start our search. We can always broaden our net at a later date. A narrower frequency signal is also easier for us to pick up above the normal noise level. Unfortunately, very narrow frequency signals mean that we have to look at millions of very narrow frequencies to find just the right one.