Because it's hard to live a faithful Catholic life, the narrow gate and all, and it's impossible to live it without the grace of Our Lord.
Instead of berating them, telling them they're going to hell if they don't do what the Pope says,
Oh yeah like that happens.
Perhaps it would also be a good thing to find out why a significant number of clergy don't accept the teaching either.
That's easy, they've lost their faith or they never really had it to begin with. They're apostate.
I suppose one question to pose, is what should be the mission of your Church? Is it to try to reach into and enhance the spiritual and human lives of the bulk of its flock, and expose them to the tools and message which will tend to assist them to be better and happier persons while on this moral coil, as a more enobling precursor to the heaven in which your Church believes, inspired by the example of the essential Christ, or to become a much more exclusive club, with the bar set so high, that it becomes more like the Marines, where only a few "good men" need apply, with the rest encouraged to take a hike? It is a choice for each to make, and that choice I think animates much of what divides around here when it comes to matters of your Church. The grand thing is that each are free to choose.
Instead of jumping to conclusions about why Catholic couples use contraception (as you did), I am suggesting we ask them and let them answer.