Although technically a word based on Rechitza would be the best choice you or I would offer, the fact of the matter is our Rabbis in general chose a much less common word for hand washing which was Netila. So why wouldn't that word be used here?
It's true that Netila is a curious word who's etymology is not so clear, but this actually strengthens the question. Once they chose this word, why not use it here?
Someone suggested that the word Netila would be associated with a washing with a bracha so Urchatz was chosen to make a distinction. This suggestion however would again intensify the question why have Rachtza to mean washing with a bracha, choose Netila!