There are several rumors surrounding the wide use of gorillas in comic books at the time:
1. Publishers believed that placing a gorilla on a comic book cover, regardless of the context or relevance, would automatically correspond with an increase in sales for that title; Irwin Donenfeld was the first to notice this trend. The claim eventually became self-fulfilling as certain comic book collectors with an eye for "oddball" comics, would single out these issues, and purchase them for their collections.
2. Due to the first rumor, it is also reported that DC Comics was worried about abusing the winning formula, so had an unwritten rule that there could be no more than one gorilla cover per month across all of its titles (except, of course, during the occasional "gorilla month").