A quick look at the Greek alphabet will show that some letters are the same as in the Roman alphabet (or at least look like a Roman-alphabet letter), while others are distinct. Let's call these two groups shared letters and distinct letters. If I'm in a country that uses the Roman alphabet (like the United States) and I go around wearing a shirt with three shared letters on it, you will probably read them as Roman letters. If I want to signal to you that they should be read as Greek letters, I have to include one or more of the distinct letters.
I suspect that the naming of fraternities and sororities is affected by the need to include at least one distinctly-Greek letter in the organization's abbreviation for the purposes of signalling the nature of the organization. And the list of fraternities and sororities at the bottom of this Wikipedia page shows fewer than five names without a distinct letter from a list of more than 100 names. Given that shared letters make up 14/24 (58%) of the Greek alphabet, there's a 19.5-percent chance that you would select three random Greek letters and end up with all shared letters. This is four times more than what we actually see.