Because, like X, there aren't any other interesting things for Z. The Romans dropped it from the alphabet they stole from the Greeks, then had to put it back in because they needed it for loan words. (Which is why it's sixth letter in the Greek alphabet, but dead last in the Latin one.)
And so we come to the end of 26 in 26. I'm mostly pleased with it, and hope to continue regular blogging service going forward.