It's becoming more and more obvious this was supposed to be released in early/summer 2013 but for some bizarre reason was held off.
Frankenweenie hit blu-ray/DVD in early January 2013 (Martin Landau), Monsters Inc then hit 3D blu-ray in February (Jennifer Tilly), Peter Pan being released almost exactly around the same time on blu-ray (10th Old Man), and then finally they delve out of home releases for a promotion for Oz the Great and Powerful which was released that March (Zach Braff). Not to mention that Wreck-It Ralph was also released on blu-ray/DVD around this time as well, given the not-so-subtle tie-in of the arcade cabinet.
Next week is featuring Richard Sherman (the composer) and Mary Poppins blu-ray/Saving Mr. Banks wouldn't occur until much later in the year... however, Oz the Great and Powerful was released on blu-ray almost exactly around the time the first Saving Mr. Banks trailer was released in June/July 2013. So my immediate guess is that Disney financed two sets of three episodes: one for the winter of 2013, and another for the summer of 2013 based around Disney digital content (remember every release had a corresponding digital release, and trailers come with that)... each one was probably supposed to be bandied with an advertisement for whatever they are promoting. Only until we see what the sixth episode advertises can this theory be truly confirmed. (my guess? an ad for the original MMW teaser which came out in July 2013)
It is, however, fairly unsettling that these episodes were produced and clearly meant to be a marketing gimmick using the Muppets to showcase attention onto other things only to be pulled, and then ironically used as a promotion for ITSELF. I wonder if Disney saw production dailies of MMW coming in right as these were being edited and finished, got worried and pulled the ads just in case the home video release needed an extra oomph in the case that the movie underperforms. (hoping no one would notice the oddly dated marketing gimmicks) It's a strange theory, but that's the only one that seems plausible.
The actual episodes are likable and cute, and kind of gives weight to the idea behind a new Muppet show.