Why? They can still maintain the gaming business. Sony and Microsoft have other venues, so why can't Nintendo?
Because, to be quite frank about it, they just can't afford it. Sony and Microsoft have massive, multibillion dollar war chests from their external divisions that have, so far, shielded them financially from the fallout of venture losses. The abysmal Vita, for example, doesn't run the risk of bankrupting Sony.
Nintendo doesn't have that buffer. They went this year, as a company whole, from a net gain (and projected profit of 100 billion yen) all the way down to a net
loss of over 35 billion. Even after accepting responsibility and slashing his salary in half, Saturo Iwata knows that he and his company are in deep trouble. The board is looking long and hard at his leadership and there has been talk of a change.
Nintendo feels that their traditional business model isn't working. Their game console is a ball and chain dragging them under and the company needs to change direction. That means not only a refocus, but cutting off dead weight and getting rid of what they view as unprofitable.
To put it simply, the Yamauchis aren't at the helm any more. Nintendo is at the behest of a board of directors that wants results. Despite the fact that consumers may think fondly of Nintendo's stock of classic IPs, the failure of the Wii U has shown the company that reliance on staples like Mario aren't going to win the day any longer. Pure game consoles- the business Nintendo's been in since the late 70s-have been bringing Nintendo less and less core profit, and they know it. The complete and utter failure of the Wii U may be the nail in the coffin.
If Samus and Zelda don't sell units- don't expect them to last. I don't expect Mario to die off, being that he is Nintendo's mascot and all, but Nintendo can't be blamed for not wanting to maintain an unprofitable group of intellectual properties.