Mainly, it's to prevent letters from running together. All capital letters are exactly the same height. But some lowercase letters have tails that project below the baseline: g, j, p, q, and y. These tails can overlap letters on the line below, making the text difficult to read. You can prevent this by spacing your lines of text farther apart, but that means fewer lines will fit inside your word balloon. By using all capitals, you can space lines closer together, and fit more words in a balloon above the cartoon character..

So that Chinese kids can read them too. Ahahahahahaha....