Because if the consciousness/mind is the ego (a permanent entity), then enlightenment or liberation is impossible. Foolish will remain foolish forever, intelligent will remain intelligent forever. Poor will be poor forever, rich will be rich forever. That theory contradicts in itself. There are no layers of mind/consciousness. Consciousness is a process or rather consciousness is no more than an empirical or functional reality that results from an assemblage of conditions. That the consciousness (citta) is the chief is also to be understood from the fact that the unenlightened are attached to it - not to the concomitants (cetasikas) - as the Atman (Ego/Self).

Moreover, when a person is liberated, one speaks only of the consciousness (citta) being liberated even though it is at that moment being accompanied by its concomitants (cetasikas) and dharmas belonging to other skandhas (factors). This is once again because these accompanying dharmas are all depended on the consciousness (citta), and when the consciousness (citta) is pure, the other skandhas (factors) are likewise pure.

We should clearly understand what do we mean by consciousness and how consciousness comes about. Consciousness comes about with the contact of eye-organ and the external object as a unit. For instance, when my eye-organ hits on a book (external object), only then the consciousness of a book is arisen. Other than that, consciousness is merely an immaterial dharma. But then again that image imprinted over again and again can perpetually co-exist in the "mental consciousness" (mano-vijnana) that recognizes that particular object whenever I see that again and it continues through out time and space; thus samsara. This consciousness can function pertaining to the three periods of time: past, present and future, while in fact, the so called "original nature" of the mind/consciousness was/is nothing but a pure impersonal dharma, that is often ineffable in its highest sense. It can be realized in deep/high meditational (jhanic) experiences, when every delusional thoughts, i.e. prapanca-s (imaginations/conceptual proliferation) are suspended or dis-communicated. However, one has to be really - i have to emphasize here - really careful not to be misled at this crucial stage, because this fine consciousness (citta) can seem like the "Ego/Self" existing throughout time. With ethereal bliss gained from the seclusion of some defilements by the near-saint-yogi, it's still plausible to attach to that desirable object thinking it is the ultimate permanent entity, while in fact, it is not. The yogi must finally go beyond even that stage in order to become fully enlightened and liberated.