Why is there no verse in Ashrei that begins with the letter nun?

Hello, 

That is a fine question, and the answer is found in Tractate Berachot (4b): because it contains the downfall of the enemies of Israel (that is, the people of Israel, expressed in euphemistic language), as it is written (Amos 5:2), "Fallen, no more shall she rise, virgin Israel." That is, since there is a verse speaking of the downfall of the people of Israel that begins with the letter nun, they did not wish to bring a verse beginning with the letter nun in this psalm. The Rashba explained in Responsa of the Rashba, Part I (Siman 49), that although there are many verses that speak of the downfall of Israel, and many of them begin with other letters, nonetheless only in this verse is it said that there will be a downfall after which there will be no recovery. 

With blessing, 

Hillel Meirs