Although most of what Stan Lee and Jack Kirby touched turned to gold in the 1960s, as pointed out by commenter Thelonius Nick, in mid-1963 they did not yet look like a guaranteed winner. Their second book, The Incredible Hulk, failed to generate sufficient sales to justify continuation after the sixth issue.
Why did the Hulk fail? Looking at the first six issues as whole I would point to several things:
1. Poor development of the supporting cast. The three backup characters are Rick Jones, Thunderbolt Ross, and Betty Ross. Yet after six issues we know nothing more about them than we had in the first issue. Rick's a teenager with a jalopy; how many of those have we seen in the comics? Thunderbolt had some potential as the first JJJ, but his daughter Betty was a typical Marvel love interest of the 1960s; only useful as a hostage.
2. No real sense that the Hulk/Banner relationship has been well-thought out. In the first couple issues the transformation takes place at night a la Dr Jekyll and Mr Hyde. That seems to have cramped the storylines, so they went to it becoming voluntary for Banner by stepping in front of the gamma ray machine. In the final issue it was changed so that emotions triggered the switch. In fairness, the Hulk was a new type of character among superheroes in that he was not voluntarily "super", so they may have been feeling their way.
3. Few worthy villains. The Ringmaster and his Circus of Crime had several more appearances, but Tyrranus (who appeared in #5) and the Metal Master (#6) were characters of limited interest.
4. In an obvious attempt to prop up the sales, there was a crossover into Fantastic Four #12. But that was the same month that Incredible Hulk #6 was published, so it was a little late to save the title. In addition, Steve Ditko took over the artwork with that issue; changing artists is another symptom of a flagging title.