Pff Metroid? Who wants that when we have like six Mario games.
New Super Mario Bros. U
Super Mario 3D World
Mario Kart 8
Mario Vs. Donkey Kong
Mario Party 10
Mario Maker
Now seriously, Nintendo needs to get off from making so many fucking Mario games (as well as Zelda remakes) and revive old franchises that have been dead for a while.
1. Those are six "Mario games" in the same sense that Gran Turismo, Rachet and Clank, God of War, Little Big Planet, The Last of Us, and Crash Bandicoot are six Sony games. Nintendo slaps its Mario Mascot on about half of its games (give or take, I haven't really counted), but most games featuring Mario on a given console are very different, so it is not as if it is one Mario platformer game and five sequels.
2. Many of those "Mario games" are really successful sales-wise. Metroid is not. Nintendo is going to follow the money, just like any for-profit company. If Sony had a mascot half as successful as Mario, they would probably put him in half of their games too.
3. Nintendo has shown a commitment to releasing Metroid games, even though they have never sold very well. History has shown that it simply is not a system-seller, so don't expect it to be a top priority. They released two Metroid games on the Wii despite the fact that the first Metroid Prime games didn't sell very well on the Gamecube, even though the first one was critically acclaimed and won a number of game of the year awards.
4. They probably want to avoid another Other-M debacle, thinking that they have made this great game that had fantastic graphics (for the Wii at least) and a deep story into the history and motivations of one of their main franchise characters, only to have it seen as doing a disservice to the series by many fans. They probably want time to work on it and make sure the new Metroid game is what Other M should have been.