Guitar masses, ecumenical services, wishy-washy new-age, save the planet for God non-sense does more harm to Christianity than does a firm stance saying: "This is where we've been for 2000 years, and this is where we're staying!" (Pope Jean-Paul II understood this hence his popularity.)
See my tagline.
I'm sure there are as many excuses as there are people who fall away.
But you can't expect catechism to compete with science. Catechism isn't about physics, it's about metaphysics. And you have to be intellectually mature enough to understand the difference.
I think Catholics need to do a better job (and they are starting to with colleges like Thomas Aquinas and a return to traditional Aquinian teaching) of showing Catholics that their philosophy is every bit as deep, profound and coherent as anything taught at Harvard or Princeton.
As a Catholic, at some point in a philosophical debate, you will simply end up having to say "Fide!". But it is surprising to most Catholics (and certainly to most non-believers) just how far into arguments about form, substance, act, potentiality, nature, etc you can go before you reach that point.
Many young Catholics were drawn to the church during the reign of JP-II precisely because they were attracted by the intellectual rigor with which he argued his points.
Compared to the lack of rigor in most of the social sciences today, it's like a breath of fresh air to them.