The problem for developing for Wii is/was that developers didn't have enough incentive to create something that couldn't be easily multi-platform. Developers are now in the business of making as much money as possible, because we live in a competitive, capitalist world. So, in a business perspective, it makes sense to develop for PC/Xbox360/PS3, because it maximizes profit for minimal effort.
That to me really explains the lack of 3rd party "core games" on the Wii, but if we're being honest here, "core games" somehow became synonymous with "HD shooter", which Wii obviously lacked. I would argue that Nintendo's AAA games are still very much core games, because they appeal to an audience of dedicated gamers. And when it comes to platforming, Mario is still the king.
