Maybe because the constellation is shaped like a scorpion, and scorpions were plentiful in the desert???
No lines in the sky, plenty of other stars and no one set the borders.
The 1006 supernova as imagined by astronomer Tunc Tezel to have been seen from a spot in what is now known as Turkey
From Wikipedia :
The 1006 Supernova was the highest relative magnitude stellar event in recorded history. First appearing in the constellation of Lupus between April 30 and May 1 of that year, this "guest star" was described by observers in Switzerland, Egypt, Iraq, China, and Japan. The Egyptian astrologer Ali bin Ridwan, writing in a commentary on Ptolemy's Tetrabiblos, has left us the most complete historical description of the supernova. He says that the object was 2-1/2 to three times as large as the disc of Venus, and about one-quarter the brightness of the Moon, and, like all other observers, says that the star was low on the southern horizon. Monks at the Benedictine abbey at St. Gall broadly corroborate bin Ridwan's observations as to magnitude and location in the sky, additionally writing that "[i]n a wonderful manner this was sometimes contracted, sometimes diffused, and moreover sometimes extinguished." This last is often taken as proof that the supernova was of Type Ia. Some sources state that the star was bright enough to cast shadows; it was certainly seen during daylight hours for some time, and the modern-day astronomer Frank Winkler has said that "in the spring of 1006, people could probably have read manuscripts at midnight by its light."
There appear to have been two distinct phases to the observation of the supernova. There was first a three-month period at which it was at its brightest; after this period it diminished, then returned for a period of about eighteen months. While most astrologers interpreted the event as a portent of warfare and famine, it is worth noting that the Chinese astrologer Chou K'o-ming interpreted for the emperor that the star was actually an "auspicious star."
A petroglyph of the Hohokam has been discovered which may be the first known North American representation of the supernova.[1]
It looks like a snake to me, even complete with a forked tongue, and coiled ready to strike, and snakes are even more plentiful in the desert than scorpians.