I think part of it is that DC periodically buys other comic companies or their characters and wants to integrate them into their main line. Fawcett, Charlton, WildStorm, Milestone, etc.
Another part is that most of DC’s big characters go back, in some form, to the late 1930s or early 1940s. Marvel as we know it today essentially started in the Silver Age with Fantastic Four, Spider-Man, Hulk, etc. The biggest Golden-Age character for Marvel is Captain America, who was frozen in ice until the Silver Age. So effectively, Marvel has fewer years of stories to keep in continuity.