I'm going to attempt to take this seriously.
To be a [game] killer, you have to do exactly what [game] does, and do it better in every way. A very rare occurence, that usually only happens in a sequel. For example, halo 2 killed Halo 1, because it diod the exact same thing only better, and it had online play.
Little Big Planet is a great game and a lot of fun, but it's nothing like either the 2D or 3D Mario games. LBP is a physics based platformer with a level editor that emphasises multiplayer and online. Mario is a reflex and combat based platformer that is realisticly regarded as isngle player. They're too different for one to kill the other.
Mario is big and recognisable. Sackboy is cute but lacks identifying characteristics. When you make the main character too customisable you're left with something that's not easily recognisable. No character comes to mind when someone mentions the Sims, but a mention of Metroid will instantly bring to mind Samus Aran. Branding is very important. Without a strong character supporting the franchise it can never be as succesful as Super Mario.
Another factor would be the time each fracnhise has been around. Nintendo has had decades to perfect what a Mario game is, LBP2 would be just two games into a franchise. Although LBP does what LBP does very well, Super Mario Galaxy does and absolutely amazing job at doing what SMG does. Maybe in twenty years the Little Big Planet franchise will be at the stage where it knows exactly what it wants to be, but as of now it's finding itself.
"Now, a fun game should always be easy to understand - you should be able to take one look at it and know what you have to do straight away. It should be so well constructed that you can tell at a glance what your goal is and, even if you don’t succeed, you’ll blame yourself rather than the game. Moreover, the people standing around watching the game have also got to be able to enjoy it." - Shiggy
A Koopa's Revenge II gameplay video