While michyprima is correct, he didn't really give any explanation as to why. Basically, the reason is that sky3ds works by imitating the behavior of a legitimate cartridge. In other words, the 3DS is incapable of telling the difference between a sky3ds and an actual game cartridge. While this means Nintendo can't break it with an update, it has the downside of only allowing code signed by Nintendo to run, as it still checks the signature. This enables you to run pirated games, as the signature is just as valid for a pirated copy as it is for a legitimate copy, but if there's no signed copy of something available (like the devmenu) it means it unfortunately can't be run using sky3ds.