New King Did Not Know Yosef

שמות א,ח : וַיָקָם מֶלֶךְ־חָדָש עַל־מִצְרָיִם אֲשֶר לֹא־יָדַע אֶת־יוֹסֵף

It is not possible for any King of Egypt to have forgotten Yosef or even to act as if he did not know him. Such deceit would be unwise politically especially since Yosef instituted the law of the “fifth”, granting one-fifth of all produce to Paroh.

The Sforno, consistent with his principle that evil befalls the Jews due to their sins, explains that this Paroh could not conceive that such a noble soul as Yosef could be related to such a base people. Hence gratitude and appreciation were no longer obligatory.