Changing your mind about something you believe deeply and truly isn't easy. In fact, it's downright hard to do.
Changing your views on something you hold to be an absolute truth, something you feel in your deepest self to be self-evident, something so undeniably and incontrovertibly true there's no contesting it... but then having the fortitude to examine it closely and re-think what you think you know, and consider all the objections as fairly as possible, realizing you do hold biases, and then strive for the best possible objectivity and finally, upon finding ample evidence / reasons to doubt yourself, consider other options / possibilities is not easy. Not by a long shot.
What it is, is downright hard.
Critical thinking, learning to be skeptical and doubt within reason, setting bias aside, striving for objectivity, all in the pursuit of the truth are skills one must develop them over time. And not everybody posses these skills. Just like not everyone can play the piano.
Of course, you know why many cannot play the piano, right? Because playing the piano is downright hard. It's a skill that one has to develop over time. And even then, after years of practice and diligence, there is no promise you'll be great at it.
The same is true of critical thinking skills. They are downright hard to develop and even harder to hone. Not everyone has taken the time to develop them. In fact, most haven't. And those who have, well, like the analogy of playing the piano, there's no guarantee they'll be any good at it. It's something you have to continually work at.