It's all to do with the way this epistemological viewpoint understands the relationships between me and the world. Ratio-empiricism inherits from its parent views the basic orientation of a thinking/experiencing subject confronted by a world of passive objects. I am the subject; everything else is an object. Now, in one sense this is a simple truism. As Kant so helpfully pointed out, it must be possible for me to attach the label "I think" to every one of my thoughts and perceptions - that is to say, I am the subject of all my thoughts and perceptions. If it were not so, they would not be my thoughts or experiences.
(As an aside - and feel free to skip this paragraph - this is actually not nearly so simple as it sounds. Kant himself ends up reducing the "I" which is subject to nothing more than a logical tag - quite literally, an ownership label which holds thoughts/perceptions together in one consciousness. The problem emerges most clearly when you consider introspection: me thinking about myself. It must be possible for me to say "I think" about these thoughts, or they are not mine. But the "I" in "I think" is the subject of the thought, whereas the thought itself is of me as an object. How "I" become an object to myself is quite difficult. Kant avoids the problem by maintaining that "I as subject" and "me as object" are completely different, the former being noumenal. Well, that's transcendental idealism for you.)
This orientation - a thinking/experiencing subject confronted by a world of objects - will get you a long way in the natural sciences. Any critique of this viewpoint cannot be absolute, but must be simply a qualification - if you like, a "yes, but..." Still, it is possible for a "but..." to raise such a fundamental question that one is forced to revisit the "yes" and reconsider it. This is, I think, one of those cases.
Because there is simply no room in this world of subject/objects for people. There is, presumably (although this concept is not without problems), one person - me - but there are no others. A person, I take it, is someone who can themselves be a subject in the same way that I can be a subject. Obviously, not a subject of my thoughts/perceptions, but a subject of their own thoughts/perceptions - another centre of consciousness.
Qualifications: obviously, there will be a sense in which another person is an object to me. And strictly speaking, ratio-empiricism does not of necessity deny that the object in front of me could be another centre of consciousness.
But ratio-empiricism does make this concept highly problematic (in both the common and Kantian sense). If knowledge really works the way the ratio-empiricist claims, or rather assumes, it does, then I am bound to treat the other person as a passive object. I am bound to approach them, epistemologically, as if they were not a person in the way that I am. The gap between my consciousness and theirs cannot be bridged in any way on this worldview. The idea of other conscious beings becomes something that is strictly beyond my ability to know: it can be thought, but not tested, and therefore lies outside knowledge.
There is no room for people in Kant's world.
If this isn't making sense, I promise it will start to come together tomorrow when I run through some of the implications as I see them...