Because it's a dumb "specific weakness" to have in the first place. It's so broad and so generic and so undefined ... and so abused.
It's almost as dumb as a character having a specific weakness to "Science" or "technology".
What good storytelling has come out of Superman being particular vulnerable to this undefinable element? None that I can think of, outside of Mxyzptlk (which forces Superman to use his mind to come out on top, which I love).
"Any doofus waving a magic wand can take out Superman because he's vulnerable to all magic"
is an actual argument I have heard before when Magic Character X would kick Superman's ass.
Magic has no rules, no boundaries and therefore makes for a stupid weakness.
For Pete's sake, just channel all that into old-school Red Kryptonite and call it a day.