Anthony Hewish discovered first one, and then a second regular pulsing source in two widely-separated parts of the sky. The scientists decided that, if this proved to be correct, they could not make a public announcement without checking with higher authorities. But preferential beaming in our direction makes little sense unless there is a message in these channels. A similar remark might apply to pulsars. But Shklovsky's principle of assuming such sources natural until proven otherwise, of course, holds.
Extraterrestrial intelligence is the explanation of last resort, when all else fails.
But even here there are interesting unexamined possibilities. One would need only a very small movable shield above a pulsar surface to modulate emission to Earth. This seems much easier than generating an entire pulsar for communications. For signaling at night it is easier to wave a blanket in front of an existing fire than to start and douse a set of fires in a pattern which communicates a desired message. 15 Sagan's suggestion was not taken up by the astronomical community.