I couldn’t say it, but Alexander Johannesen could: libraries are the last bastions of the “not invented here syndrome” (scroll down just a bit, you’ll find it).
Between Alex’s post and mine, I don’t think there’s much to say except this: there may be five programmers in the world who know how to work with Z39.50, but several thousand who can build an Amazon API-based application in 15 minutes. What technology do you want to bet on?