It could be anything. Optimistically, perhaps they are taking the criticism of the 2016 Touchbar MBPs to heart and redesigning them to be more practical. Doesn't excuse the lack of updates to the Mini and Pro though.
Operating systems are sticky. If there is no up-to-date Mac hardware when I want a new computer, I will switch to Windows/Linux and not come back. Apple's strategy is one for slowly killing the Mac (but they can't intend that because they keep pumping out new versions of macOS like clockwork). I can't detect any logic in what Apple is doing.