WE DON’T USE ALL CAPS ONLINE BECAUSE IT IS SHOUTING AND IT IS HARD TO READ!
We read not by looking at individual letters in a word. We read by recognizing the shape of how a word is created by its lowercase letters.
ALL CAPS CREATE WORDS WITH NO UNIQUE INDIVIDUAL LETTER SHAPES!
Mixed case for words works best for understanding each other via a text medium.
A friend of mine sent me the following in email and this example takes word shape recognition a step further:
I cdnuolt blveiee taht I cluod aulaclty uesdnatnrd waht I was rdgnieg.The phaonmneal pweor of the hmuan mnid Aoccdrnig to rscheearch at Cmabrigde Uinervtisy, it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoatnt tihng is taht the frist and lsat ltteer be in the rghit pclae. The rset can be a taotl mses and you can sitll raed it wouthit a porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe. Amzanig huh?
It appears we may not even care in what order the letters make up each word — we see the word shape even if the letters of the word are jumbled: Wlid, eh?