unfinished work on crypto driver
Well, as far as I can see, the problem with mt7628 is that it is low-end now, while formally it is improvement over mt7620 in many aspects, the latter was not that "low-end" for its time. So, the lack of interest for developers. 
Although there obviously are many niches where mt7628 could be a quite adequate choice.
Some hope still lives in me 