There's theories why Superman is weak to magic, but nothing definitive, ranging from his heritage to magical radiation.
Superman’s best-known weakness is Kryptonite, but he is also vulnerable to magic. There are a few reasons people believe this is the case, and it has often been used against the Man of Steel. We’ll explore some of those occasions and the possible reasons the world’s greatest superhero is no match for magic.
The first reason postulated for Superman‘s vulnerability to magic is that Kryptonians have no experience with the supernatural or the occult. Krypton is famously a scientific planet, even to the point of worship, as Jonathan Kent mentions to his son in The Adventures of Superman #500. To some observers, this means that Kryptoians have not developed the skills necessary to combat magic.
However, the problem with this reason for Superman’s vulnerability to magic is that Superman was not raised on Krypton and does not carry that cultural baggage. He also could conceivably have developed such an ability during his time on Earth. Some time away at magic school at some point in his life could therefore cure him of this shortcoming or at least make him better prepared.
Since that never happens, Superman must have some other reason for being weak to magic. Another postulation is that the radiation and forces from beyond our dimension involved in magic are able to penetrate his force field—yes, he has a force field. First alluded to in John Byrne’s classic Man of Steel #1 in 1986, Superman’s electro-chemical aura gives him his incredible invulnerability and can even be extended with some effort.
In 1987’s Action Comics Annual #1, Superman and Batman fight some vampires, and Superman notes that one of the magical creatures is able to breach this field, lending credence to the idea that this aura is penetrable by magic. However, the aura was a much later invention, and the character’s magic vulnerability goes back decades before its first mention. As a retcon, force field vulnerability works to some degree, but it doesn’t really explain Supes’ weakness to things like mind control and hypnosis.
The third explanation for why magic can overcome Superman is the simplest and probably the best: he’s just not invulnerable in that way. To explain this, let’s look at another of the Man of Tomorrow’s oft-cited weaknesses, red sun rays. In fact, the red sun of his home planet, Krypton, would not hurt him at all; it simply wouldn’t recharge the powers he gains from Earth’s yellow sun.
Superman’s powers come from his exposure to the yellow sun of Earth, which enhances his strength and speed, allows him to fly (see the book The Science of Superman for a nifty explanation as to how), and gives him X-ray vision and heat vision for . . . reasons. All of those are “powers and abilities far beyond those” of mere mortals, but they are all physical powers caused by a biological response to an environmental condition. They have nothing to do with the magical realm, nor should they be expected to do so.
Therefore, Superman isn’t weak to magic per se, not in the way he has a specific reaction to Kryptonite. Instead, he simply has no more defense against it than anyone else because his powers aren’t magical. So, he can become enchanted by a sorcerer right along with Batman, Wonder Woman, and everyone else in the Justice League.
This is what allows Star Sapphire to use a magical necklace to make Superman her slave in 1973’s Superman #261 and for the hero to be hypnotized on multiple occasions. Sure, hypnosis isn’t magic in the real world, but it may as well be in comics, and it also shows his vulnerability in the realm of mental powers while his physical body is still impervious. It is also frequently listed as part of his magical weakness.
While Superman is not invulnerable to magic, the question remains open as to whether he can be killed by it, and it seems he has a mysterious backup plan to prevent that from happening. In 1939, in Superman #235, he battles Felin Nyxly, who uses a magical tool called a Devil’s harp to slow the hero down and overpower him, threatening to kill him. The harp does its job, and Superman seems to be near death until he is saved.
Superman’s salvation comes in a shadowy, silent image that looks like himself. The apparent alternate Superman breaks the harp and the spell, saving what might be his own life, but leaves without ever identifying himself. Could Superman have traveled from another time or dimension to save himself? Over 80 years later, that mystery remains to be solved.