Great Christian thinkers of this century such as C. S. Lewis, Francis Schaeffer, E. L. Mascall, and Karl Barth didn't become Catholics. Were they too smart to fall for Catholicism?
Faith is a gift from God, not merely the product of a syllogistic chain. It's difficult to explain on purely rational grounds why a given individual does or does not accept Catholicism (or Christianity in general, for that matter). Brilliance doesn't protect a man from the effects of bad thinking, his feelings, his cultural background, or ethnic prejudices.
Any one of these things could prevent someone from fully opening his mind and his heart. On the other hand, all of these together may not be enough to keep someone from the faith--God works in strange ways, you know.
There's little doubt the men you mention made great contributions to the cause of Christ in this century. But this doesn't mean they couldn't have missed something--something key or central--in their understanding of Christianity.
Take C. S. Lewis as an example. A former pupil and long-time friend of Lewis's, Christopher Derrick, noted in C. S. Lewis and the Church of Rome that while Lewis was a creative defender of Christianity in general, his reasons for not being a Catholic, to the extent Lewis made these known, were pedestrian.
This led Derrick to assert that Lewis's non-conversion to Catholicism was only partly due to intellectual difficulties. Lewis's personality and his Ulster Protestant background were also involved.
Despite his own failure to embrace Catholicism, though, Lewis has been responsible for a great number of other Protestants, particularly Evangelicals, coming into the Catholic Church. Sheldon Vanauken has spoken of Lewis as Moses--leading people to the promised land without himself entering into it.
By following his theological principles and insights, Lewis's readers are often able to see what he, given his mental grooves and prejudices, was unable to see.
Much the same could be said of the others you cite, although the specific.aspects of their thought, as well as the non-rational elements which contributed to their reasons for not embracing Catholicism, would be quite different from Lewis's.
Even if we could take all these factors into account and weigh them properly, which we can't, there's still the main element, which remains mysterious: grace. We have no scales at all in which to weigh it.