Because that is a huge risk for a company that has tried to minimize risks since the Nintendo Wii. If Nintendo puts out a $200 ARM console and it's not a huge success then they probably still made money per unit and they are set to move to a Sega role with easy ports of all that console's games to mobile devices. That path has a clear future. If Nintendo tries to push a top of the line console- aka the kind where they lost a little money per unit the first year or so- if it flops they are FORCED to move to a Sega role without billions left in the bank. That path is unclear.
I mean, the Japanese market is pretty much gone so Nintendo can't rely on that. To compete with the PS4 or Xbone straight on means Westernizing everything to appeal to the largest market that is left. To beat either company at their own game would require some cash spent on the hardware, online play and marketing. You pretty much need Nintendo to be bought by someone like Apple for them to have those kinds of resources. Moving forward with a top tier console is a huge risk without that sort of backing because they probably will have to cut a leg off of that stool (no decent marketing, or decent online play for example).
Plus there is the fact that in order for a console to succeed Nintendo needs help from the third party developers that it has pissed off since the 1980's. They can barely get out simpler Wii U games on schedule now, there is no guarantee that their first party games can carry a more powerful console when each game has an even LARGER budget and resource commitment. And they need more than ports to make their console a real value proposition, they need exclusives made either by them or a third party. That is asking a lot out of 2015's Nintendo.
Honestly I will almost be sad if Nintendo does make a PS4 killer. It will be the company's Dreamcast almost certainly. Give us the $200 ARM box Nintendo and stay alive another decade.
What I don't understand is what will this new ARM console have that the WiiU doesn't have? It's going to be some stupid new gimmick. They need to focus on just making good games. The Wii was successful because it was a good gimmick for the time. I played it for the first year and it mostly collected dust after that. But it sold amazingly well. I think you can play the gimmick card only once. People get wise to it very fast.