Some of the answer is copyright renewal and customer awareness. (This is why we sometimes get Mini-series and Essential volumes of characters/names that wouldn't stand a chance in today's market. Like the
Legion of Monsters series from 2007 and 2012)
With the Legion of Monsters, the title banner was only used for one issue of Marvel Premiere and an issue of a Marvel's b/w magazine series. However, in order to properly trademark a title banner, a company has to pay some money the first time it's used. During the 70's, many comic companies neglected to do this at times. They remove any lingering question about ownership by publishing a new product with the same title.
That answer is probably a little more Inside Baseball than you wanted, but there you go.