system
Given the following post, respond as if you're commenting on Reddit:
human
Understanding the Wow! signal: Why does 1420 MHz point to an artificial source?

Wikipedia (Robert Krulwich, NPR, May 29, 2010) states "[...] The frequency 1420 is significant for SETI searchers because, it is reasoned, hydrogen is the most common element in the universe, and hydrogen resonates at about 1420 MHz, thus extraterrestrials might use that frequency on which to transmit a strong signal."

Why would aliens choose this frequency if it is so closely related to a natural resonation? For drawing attention, wouldn't it make more sense to transmit a signal in a frequency that does not or rarely occur in nature?

Edit: Thank you KToff and tliff for the reasoning. 

So I assume when you are scanning / listening in the eletrocmagnetic spectrum you have to pick a frequency / frequency range. In case of the Wow! signal the received frequencies were 1420.356 MHz (J. D. Kraus) and 1420.4556 MHz (J. R. Ehman). 

Follow up questions:
- How wide is the range SETI is (was?) listening?
- What is the significance of the Hydrogen frequency over others? Percentage-wise in the case for SETI listening.
gpt
The Wow signal was not *on* the hydrogen line but near it.

Of course the mere frequency could also occur in nature. However, anybody making astronomic observations in the electromagnetic spectrum will be looking at the Hydrogen line anyways. So if you send it in that band, you have a high chance of being detected by someone who is looking.