I'd guess because it's a number 1.
Chamber of Chills #1, Combat Kelly #1, Hero for Hire #1, Marvel Premiere #1, Marvel Triple Action #1 are all pretty well represented in the census. Even Night Nurse has 15 copies in 9.6 or better.
By that time in 1972, there were a fair number of people buying multiple copies of first issues, especially any Superhero or Kirby book.
I've heard that Conan #1 was one of the first books to get the attention of speculators and purchased in large multiples. I'd guess that Iron Man #1 and Sub-Mariner #1 might fall into that category as well, even if they're not actually debut issues.