Why did the author of Metzudat Tzion never write an explanation of difficult words in the chumash, as he did for most of the Nach? There are hard words in the chumash that require explanation even for a fluent Hebrew speaker.
As explained in the source referenced here he started with Navi due to the poverty of a good collection of Mefarshim, Rashi on Nach is a more advanced pirush that doesn't explain the words the way his Pirush on the Torah does and there was a general lack of expert knowledge on Nach to learn from (I would say that part hasn't improved).
So in other words, he focused on Naviim and Kesuvim because he saw that as in the most need. He never finished with Nach, so going back to doing the same on the Torah never happened, if the author would have found it worthwhile.