No reason really it seems.
Superman’s official birthdaywas established in the Silver Age when editor Julie Schwartz declared that Superman’s birthday was on February 29th in a number of letter pages.
Created by Jerry Siegel and Joe Shuster, Superman first appeared in ACTION COMICS #1 in June 1938. The popularity of Superman launched the superheroes in comic books. If not for Superman we would not be enjoying comics today. The character turns 74 this summer.
In the comics, Superman’s (aka Kal-El) birthday was initially stated to be in October as stated in ACTION COMICS #149 in 1950.
Clark Kent and Superman have separate birthdays. Clark Kent’s birthday is the day the Kents (the Earth couple who adopted him) found baby Kal-El. The first mention of Superman recognizing his Earth birthday was in ACTION COMICS #241 in 1958. SUPERMAN #263 (1973) puts Clark’s birthday on June 18th.
Superman’s birthday is established as being on February 29 in an actual story in Superman Annual #11 in 1985. A “TIME” magazine story in 1988 stated that Superman’s birthday is February 29 and that year DC Comics hosted a party for the Man of Steel.
February 29 is Leap Day, which occurs once every four (Earth) years. Kryptonian years are different than Earth years—there are eighteen Kryptonian years for every twenty-five years on Earth. So how old is Superman?