Dust is a factor, but so is cold and diminishing solar. My guess is that the dust isnt a problem until late in the season, and by then they hope to have gotten all the data they came for.

IIRC, one of the reasons the MER rovers lasted so much longer is the dust clearance from wind, which was much more than expected. Perhaps Insight counted on more clearing than they got.

I'd just go ask their Project Scientist on Twitter. @MarkPanning