From what I hear, he was the most popular among the O5, which explains a few things, he's the first of the O5 who got an origin story, after X-Men got cancelled, he got his own stories in Amazing Adventures while the X-Men were in limbo, and he got to join the Avengers around the time the X-Men returned.
Reading the comics themselves, it's easy to see why he was popular, Cyclops, Jean and Angel were kinda bland, Iceman was a less interesting version of Human Torch, Hank was his own thing after his sudden personality shift (He was just a copy of Thing in the first three issues), and he was generally pretty amusing, he's the only character from the Lee/Kirby era who I found interesting.