There are two reasons why acquiring true knowledge of the world is difficult. If we remember that human knowledge involves both the intellect and the object thereof, we see two potentials for failure. One is that the object itself is difficult to know; the other is that there is some failing on the part of our knowing power, i.e. our intellect.
Difficulty from the object itself: Plato and many of the pre-socratics agreed that the sensible world was composed of things that were constantly changing. Matter, motion, time, etc. all contribute to the indeterminacy of an object. The more that an object is indeterminate, the less it is knowable. The mind knows being, and being is opposed to becoming. Therefore, inasmuch as an object is becoming something else, a state all material substances share, it cannot be perfectly known.
Difficulty from the intellect: Conversely, it is not the case that we know the immaterial and insensible perfectly merely because they lack matter and motion. We know this because if this were true than that which is most knowable in itself would be most knowable to us; we would have direct knowledge of the immaterial. Humans do not possess this.
Between these, the second difficulty is the chief one. The human soul is the act of a body, it cannot know without the body, meaning without sensation. Therefore we are limited to direct knowledge only of the sensible and bodily. These things are the least knowable things (see the first difficulty), so we are naturally inclined to have knowledge of the least knowable and cannot have direct knowledge of that which is most knowable in itself. The human intellect, therefore, is the lowest of intellects.
The nature of our knowledge of immaterial and sensible things cannot be identical the the nature of our knowledge of the bodily and sensible.