A. The Talmud (Brachos 4b) asks the same question and explains that Nun stands for Nofel (fallen) which is something David (author of Ashrey) wanted to avoid. But you asked further – aren’t there other Hebrew letters that stand for negative words? Are you going to disqualify all letters because of what they may stand for?
Here’s a nice life-relevant explanation: King David wanted to include “fallen” in Ashrey, after all it’s ups and downs are a fact of life. But he didn’t want to start off a verse with Fallen or even hints of it. Instead he begins the next verse (starting with the letter Samech) “G-d supports the fallen.” It’s OK to talk about falling down, as long as it’s in a supportive and hopeful context.