Way back when in journalism school, we were treated to data showing the time it took to read text in different formats, as well as how far people would read before abandoning a particularly formatted text and how readily the content was understood and retained.
Unsurprisingly, TEXT TYPED LIKE THIS TOOK FAR LONGER TO READ, AND MOST PEOPLE LEFT OFF AFTER LITTLE MORE THAN A LINE OR TWO.
Text formatted within our normal conventions was more quickly read, comprehended, and retained.
Text Formatted in This Manner was somewhere in the middle in terms of reading speed, comprehension, and reader persistence; perceived as more formal than the second sample above, it was thus seen as appropriate for titles and headlines.
We were taught that all upper case should be reserved for headlines announcing wars and assassinations.
Besides, it's ugly.