Since repeatability is paramount to verifying the veracity of an alleged alien radio signal, why was the Arecibo message only sent once?

It was an SOS signal. Help did not arrive in time.

I read that there was a huge controversy about whether to send the message in the first place, because the world was not equipped (that is, hadn’t planned) for what happens if we somehow got a response.

My surmise is that they got the OK to send it once, but then the cowards kicked in and prevented the experimenters from sending another.

And it has been almost 50 years with no answer.

It was part of the celebration for a ribbon cutting at the Arecibo radio telescope and more of a technical show off. It was supposed to demonstrate the kind of signal detectable by said radio telescope. It was directed at a star cluster 20,000 light years away so an answer is not coming for at least another 39,950 years.

Here is an excellent article describing the message and how the destination was selected.
The 3 minute message was sent on Nov. 16, 1974.

And Here is the message.

“The message consists of 1679 bits, arranged into 73 lines of 23 characters per line (these are both prime numbers, and may help the aliens decode the message). The “ones” and “zeroes” were transmitted by frequency shifting at the rate of 10 bits per second. The total broadcast was less than three minutes. A graphic showing the message is reproduced here. It consists, among other things, of the Arecibo telescope, our solar system, DNA, a stick figure of a human, and some of the biochemicals of earthly life. Although it’s unlikely that this short inquiry will ever prompt a reply, the experiment was useful in getting us to think a bit about the difficulties of communicating across space, time, and a presumably wide culture gap.”

From Wikipedia-
“Since it will take nearly 25,000 years for the message to reach its intended destination (and an additional 25,000 years for any reply), the Arecibo message is viewed as a demonstration of human technological achievement, rather than a real attempt to enter into a conversation with extraterrestrials. In fact, the core of M13, to which the message was aimed, will no longer be in that location when the message arrives. However, as the proper motion of M13 is small, the message will still arrive near the center of the cluster.According to the Cornell News press release of November 12, 1999, the real purpose of the message was not to make contact but to demonstrate the capabilities of newly installed equipment”

Sent in 1974. Only 24,956 years till it gets there!