The company is having internal discussions about what to do with the series and wants to serve fans of both traditional and Prime styles of play.
http://www.nintendoworldreport.com/news/37837/nintendo-hopes-to-have-metroid-news-to-share-in-the-near-future
Nintendo is talking about what to do next with the Metroid series and hopes to have news to share on that front soon, according to a Kotaku E3 interview with Nintendo Software Planning & Development head Shinya Takashi.
When asked about the current status of the series, Takashi said, “So it has been a while since we released the last one and we're having discussions internally about what we can do next.” He noted that they “have two different types of Metroid games,” with these being the “traditional style” and “the Prime style.”
“We feel that we do need to take care of both of these styles of play,” he said. “And the hope is that at some point in the near future we'll be able to share something about them.”
A bit later in the interview, Takashi and the also-present Shigeru Miyamoto were asked to choose which style of Metroid was their favorite. Takashi likes them both, but Miyamoto responded, “I have the New Super Mario Bros. series [for 2D], so I like Prime. I think there’s still a lot of new things that could be done with the Prime series.”