All-uppercase distorts the shapes, making them unfamiliar to ESL readers who have less practice. You must know that famous meme about how you can read English perfectly fine if the letters (besides first and last) of each word are scrambled: "Aoccdrnig to rscheearch at Cmabrigde Uinervtisy..."

Granted, I'm not an expert in this area. I'm actually just relaying what an ESL ex of mine told me. She hated whenever she had to read things like "Calvin and Hobbes" which use all-caps, for this exact reason. Come to think of it, she was Japanese, I wonder if it has to do with growing up with a logographic writing system.