A person could theoretically study a number of Torah books, each of which pertains to a different area of Torah, so that, by accumulation, he has "covered" all aspects of the Torah.
But these remain distinct works, each offering a vision of Torah which encompasses only a part – either a large part or a small part – of the whole.
All other Torah books, even if they are large in scope, are still about details of the Torah.
Only Rambam's Mishneh Torah is about the whole of Torah.
This is what is unique about the Rambam's work—not just about Mishneh Torah as a whole, but also about every individual chapter in it, since the entire work is constructed as a holistic view of Torah.
When you grasp a piece of something, even when you grasp that piece entirely, you only have the part.
But when you grasp the whole, even if you only grasp it partially, the part you grasp contains the whole within it.
–Rabbi Adin Steinsaltz
From "A New Epoch in Torah Learning," an essay by Rabbi Adin Steinsaltz