That majority is hard to shake because the result of that majority is generations raised on Christian traditions, most of which are then reticent to completely let go of, for fear of how that disrupts their oldest and deepest philosophical understandings of existence which significantly shaped their worldview.
It’s hard to admit you’re wrong, ever, even over small, stupid things. Now take something really big, something fundamental, something you’ve held onto for your whole life. Admit to yourself that you’re wrong about that.
Evidently, most avoid crossing that bridge, at least entirely. Most people who lose their religion start by regressing to a vague form or theism. Then deism. Many of them stop at agnosticism, refusing to completely let go of that sliver of a chance that they could have been kinda, slightly, in a distant way a little bit right to believe. Some of them eventually move on to atheism. Some of them rip off the bandage and skip that step entirely.
For many, letting go of religion is a slow process. The mob that is society moves especially slow.