Why?

I do not want to sound like I am a little right of Attila the Hun but I would like to raise the point of Pius V's Apostolic Constitution, Quo Primum. I know it is a favourite of many traditional Catholics (and, dare I say it, many Anglo-Papists). Here is a small quote from a post on Canterbury Tales, Why isn't Quo Primum in effect?:

Here's my question. There has to be a good answer from Pope John Paul II or even Pope Benedict XVI as to why Quo Primum is not effect perpetually, despite its apparent claim to be so. Anybody, know how Roman canonists explain the Missal of Paul VI?

To give the question a wider airing I post it here. Any thoughts?