It goes clear back to the old days of the Bulletin Boards on the early Internet. It comes under the heading of "Netiquette". All caps are considered shouting, and are poor manners and poor taste. Most of us have been around a long time online. Perhaps you were unaware of that, and if so, consider yourself enlightened. It is still considered bad form, poor manners, and very rude to do so, in forums like this, or even E-mail. We have other tools for emphasis, like bolding the word, or underlining it, or putting it in italics, although here that is usually the equivalent of a quote.