Why did Pope John Paul II not accept Cardinal Law's resignation back in April, instead of waiting until now?
Mark Shea offers come excellent thoughts to answer this question (here and here), pointing to the reality that the Holy Father is the Bishop of Rome, not CEO of Catholicism, Inc., i.e. that JPII thinks in Gospel terms, not political terms.
I would also highly recommend reading Sean Gallagher's comments made in response to this post by locdog, calling for John Paul's resignation. Sean makes excellent points about a proper understanding of the structure of the Catholic Church and the relationship between the pope and his brother bishops.