Why is pNA the leaving group on all of the Chromogenix substrates?

A good chromophore must be readily cleaved by and dissociated from the enzyme. The color must be strong to allow detection of low enzyme activities, but should not interfere with the color of other reactants or impurities in the reaction mixture. It should be water soluble and have low toxicity. The chromophore para-nitroaniline (pNA) fulfills most of these requirements. It is therefore the most common choice of chromophore.