Why Wasn't Pope John Paul II Able To Ban Catholics From Using Contraception, And Did He Try?

He did.  I mean, effectively.  Within the powers that he has, which are only there as long as Catholics recognise and respect them.  Since 1968 Catholic Pontiffs have been repeating the mantra that Catholics mustn't use contraception -- the current Pope is the first to try to find a loophole that maybe condoms might be okay if one partner has a deadly STD (e.g., AIDS).  Gosh, revolutionary thinking, or what!

NOW, maybe you mean, why couldn't PJ II truly "make" all Catholics obey his edict that using contraception was sinful.  Why are birth rates so low in some European Catholic countries; why are so many Catholics everywhere evidently ignoring the pronouncements of successive Popes on the issue of birth control?

Because they don't want such big families, and they don't want to be abstinent, AND they have Free Will. Which is a basic precept of the Christian Faith (inherited from Judaism, shared with Islam).  It's the responsibility of the individual to accept the teachings of these religions.

Also, the Catholic faith (along with Judaism) is big on the idea that all of us are sinners -- even if we try to be perfect, we are merely human and will sometimes blow it.  So in some ways Catholicism can be very "Life and Let Live" -- keep striving, but it's understood that nobody is going to get 100% of their religion right, even though they should try to.  Some people take that as a green light to pick and choose.  Others just shrug their shoulders and know it's unrealistic to live ALL of life exactly as the Church tells them they should.