Really, honestly, in the nicest way possible: anyone who doesn't think MMW underperformed at the box office is absolutely delusional. It made 10 million less WORLDWIDE than the predecessor's DOMESTIC total. I don't think any Disney execs were content with that.
Instead right now, what's happening behind the scenes is that they are focusing on pushing the movie BIG on DVD to see if the Muppets will have better traction on the home video market. (I've already seen TV spots and a lot of online advertising, not to mention the Disney Drive On series that they've apparently had in their back pocket for a long while) After that they'll likely review other options that are anything but a theatrical feature.
My best guess is that Disney initially saw after the success of the 2011 Muppets is that they had their answer to Sony's Smurfs in a cheap, nostalgia-based family franchise. But now they're probably just revising their intentions. And as I said in an earlier post: I'm totally, totally cool with that.
Between the 2012 Muppets Blu-Ray release and the 2014 release of MMW, the characters and franchise sort of went into mothballs for two years without a peep. That would likely happen again with a third Muppets film. I'm excited to see what they'll do with the characters next.