Because, as the currently reigning pope has himself pointed out, “the pope is not an oracle”. Of course, when the pope teaches something from the deposit of the Faith, Christians are obliged to believe. But when he publicly expresses a personal belief in something non-essential (such as a private revelation), I do not understand why it should be imagined that any other Catholic should feel compelled to conform his personal beliefs on the non-essential matter in question to those of the reigning pope. To suggest that one should seems to me the odd position to take, especially when you consider that the next pope could conceivably have a very different take on the matter in question.
Now, I think one can say that it might be reasonable for a Catholic to, perhaps, take a second look at something of this nature if the pope deems it important enough to publicly comment on or write about it…if he cares to and if he has time to and if he feels motivated to bother with it. But if, after doing so, he concludes “nnnn…no. With all due respect to His Holiness, I still don’t believe in that, I’m afraid; it just doesn’t add up, from my perspective,” his position should not be regarded as an odd one, I think, but as a carefully considered one.
The pope, of course, as a human being who is not an oracle, will have many, many personal opinions about many, many topics and issues that are not fundamental to the Catholic Faith. No Catholic whould even be expected to attempt to discover what the pope’s personal opinions are on any such subject, much less to conform his personal opinions on any of those subjects to those of the reigning pope.
Remember, too, that we live in a media age of prolific pontiffs who enjoy expressing their opinions and publishing them. Five centuries ago, however, the average Catholic might not even have had any concept of what the pope looked like, much less what his opinion was on the latest private revelation of interest. And that was just fine. There was no need for the average Catholic to know what the pope’s opinion was on that matter, or even to be aware of the private revelation, itself.
When the pope speaks to us as bishop, teacher, and father, it is not his personal opinions that bind us, but the Christian Faith which he reiterates to us. When, as a devout Christian like any other, he expresses his personal opinion–however publicly–concerning a private revelation, he might as well be expressing his opinion on the existence of UFOs for all that it compells individual Catholics. We are not even obliged to discover what Benedict XVI believes about Fatima, much less to share those beliefs.