According to Beis Yaakov, Esther sent Hasach “on” Mordechai instead of “to” Mordechai as a sort of passive aggressive move since she was blaming him for the decree against the Jews. After all Haman was Mordechai’s slave. As such, Mordechai had the legal ability and responsibility to confiscate any purchases of his slave, especially here, where the purchase was the very life of the Jews.
Perhaps another action Esther blamed on Mordechai was his original refusal to bow to Haman.
R’ Moshe Dovid Valle points out that the initial letters of the phrase “al mordechai l‘daas” (“on Mordechai to know”) are an acronym that spells out the word “amal” (“labor”), which usually represents the negative, human desire to do wrong. In other words, Esther was pointing out to Mordechai the spiritual cause of the current problem faced by the Jews.
Perhaps another reason for this unique turn of phrase is the verse’s attempt to demonstrate a proof that Daniel (if he is Hasach) is Mordechai’s superior.