I believe PTC bought Mathcad believing it could be easily made into super engineering calculator embedded into Creo (nee Solid Works.) To do that they had to rebuild the user interface, so they launched the "Prime initiative."
It turned out that
1) It was a bit larger job than they originally thought.
2) There was this pesky group of users of the old Mathcad that kept complaining about things that they used to be able to do and couldn't in the newer versions.
3) Somebody (in house) convinced them that adding a bunch of new features was a good idea. This diluted an already strained effort
The obvious delay in new releases and revisions has suggested (see other posts) that somebody is revisiting the cost/benefit study on this whole effort.