Haven't found many outstanding answers though.
R' Shlomo Zalmen zatza"l answered that although we are not machmir throughout the year to wash on veggies dipped in water, on Pesach, due to the holiness of the night we are machmir on ourselves and wash our hands. Therefore there is a play on words at work here:Kadesh Urchatz! Make yourself holy and wash!
Other than that I have only found that it was inserted to rhyme better with the rest of the simanim, urchatz/yachatz, ruchtzuh/matzuh, korech/orech.
Hope this helps!