Adding to what Stewbacca posted, there are a couple things that seem to need pointing out. Just within the context of the Latin alphabet, the transition between having an all-caps system and the direct progression of advances that led to having a majescule/miniscule system like we have today took about five hundred years (the Uncials begat the Half-uncials begat the Carolingians). This span of five hundred years was one in which the nature of literacy changed, and though almost everyone who was literate was clergy, the clergy was really interested in getting the bible and other religious texts written as quickly and neatly as possible to aid in the dissemination of their ideas.
Additionally, the nature of writing changed: instead of needing an alphabet that could be flexible enough to work not only on stone, wax, or paper* (and here with brush or quill pen), almost everything was being done on paper with pens.
In any event, it's not that text in uppercase was too difficult to read before the advent of lowercase. Rather, it's that once we saw the benefits of the more varied letterforms (ascenders! decenders! clear visual distinction between sentences!) we found that all of a sudden it was easier to read long texts than before.
* by 'paper', I mean everything from pulp paper to papyrus to animal skin vellum or parchment and any other variation on this theme. We use sticky notes; ancient Romans used wax tablets.