Mario's early rise to stardom drew a lot of notice, but nobody took it to heart quite like Sega. Having lost some fiercely contended market share to Nintendo, their response was Sonic the Hedgehog - an edgier, faster spin on Mario that, paired with the proper marketing campaign, would position the Genesis as the edgier, faster alternative to Nintendo's Super NES. It worked, and despite the league of half-baked imitations spawned in the aftermath - Bubsy and Aero the Acro-Bat, for example - the competition for this pair of rival mascots remained largely nonexistent for years.
But if recent console generations are any indication, securing a foothold in the early years of gaming's history isn't enough to guarantee a receptive audience. A rapidly expanding industry and a maturing demographic with lots of cash to burn created a space for the hardcore gamer, one that likes guns and blowin' stuff up and has little time for the "kiddy" experiences once commonly available. By the advent of sixth-generation consoles, one might notice a commonality throughout the most commercially successful games: a gritty sort of realism complemented by characters with a developed back story and a sense of context. Marcus Fenix needed a history and was therefore created as a soldier tossed in prison for dereliction of duty, his characteristics and context shaped by the world that surrounded him. This is something Mario and his progeny fundamentally lack.
The choice for developers of many beloved franchises seemed simple: adapt or risk irrelevance in an evolving industry. Series reboots aimed at bringing characters up to speed, however, didn't always fare well; one need only mutter aloud "Bomberman: Act Zero" or "Sonic 360" to incite a riot among fans of either franchise. Undoubtedly awful gameplay notwithstanding, a common criticism of both of these games was aimed at the complications of surrounding characters with more "mature" worlds. Sonic kissing a human princess just seems weird.
Nintendo has handled Mario differently. The franchise hasn't so much changed as it has expanded; Mario's universe has become progressively more detailed with every new game, especially by those that take creative liberties with narrative and character design, like Paper Mario or Mario & Luigi: Superstar Saga. Though the stories in these games are substantially more detailed and even occasionally dark, it's never without the comic relief and self-referential humor to remind the user what he's playing.