Why do the characters in The Stroyteller by Llosa struggle to understand the Machiguenga Indians culture or any culture that they are unfamiliar with?
"Why do they struggle to understand?" can be taken two ways. It can be taken as: Why do they make the effort to understand? It can also be taken as: Why do they find it very difficult to understand? Since your meaning is not clear, I'll address a bit of each.
Using Saúl as an example, some people make enormous efforts to understand cultures they are unfamiliar with because of the belief that the modern world has become to restrictive, to impersonal, too narrow-minded, too unaccepting, too scientifically and mechanistically focused, too isolated from the natural world.
... they have a deep and subtle knowledge of things that we've forgotten. The relationship between man and Nature, for instance. Man and the trees, the birds, the rivers, the earth, the sky. Man and God, as well. We don't even know what the harmony that exists between man and those things can be, since we've shattered it forever.
So Saúl and those with the same affinity in their souls try hard to understand the world from the logic and perception of other cultures in order to (1) find their own human connections, like Saúl did, and to (2) help preserve the way of life of the indigenous people and (3) to bridge the gap of logic and perceptions between the natural cultures and the modern culture. This last is often motivated by the realization that so many natural cultures and languages have been smashed of have already died out altogether.
We know now what an atrocity bringing progress, trying to modernize a primitive people, is. Quite simply, it wipes them out. Let's not commit this crime. Let's leave them with their arrows, their feathers, their loincloths. When you approach them and observe them with respect, with a little fellow feeling, you realize it's not right to call them barbarians or backward.
On the other hand, some people find it so very difficult to understand other cultures because they have strong and deeply emotional, psychological, intellectual and ideological (i.e., the system of beliefs that determines what is valued by a set of people) beliefs, systems of logic and systems of perception that determine the appearance of what they see and that excludes acceptance of what does not accord with their systems of perception (as just described). As a result, that which opposes their own perceptions must be changed for emotional, psychological, intellectual and ideological reasons to that which does accord.
Here is the conclusion to this thought process: Since this perception must prevail because of its internal definition and construction, they find it hard to understand other cultures because such understanding would deeply conflict with their own perceptions and would, of necessity, require respect and tolerance for the other culture. True understanding leads to true respect and tolerance for other ways of perceiving through other systems of logic, perception and belief.
Saul's fears, [of] that evening, were becoming a reality. Like other tribes, the Machiguengas were in the very midst of the process of acculturation: the Bible, bilingual schools, an evangelical leader, private property, the value of money, trade, Western clothes, no doubt ... Was all this a good thing? Had it brought them real advantages as individuals, as people,...