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原文如下，有興趣的朋友可看看。
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Why is captioning almost always in uppercase (capitals)?
Numerous studies have shown that mixed-case text is easier to read than all uppercase. Virtually all captioning in North America is done in uppercase only, however, because the resolution of the caption decoders caused some pretty ugly lowercase letters. For example, there are no descenders available for the lowercase i, j, g, q, and y. This means that these letters are pushed up on the line relative to the other letters, making it harder to read.

Some of the newer televisions, however, have very attractive lowercase letters with true descenders, but this can't be assumed as a standard.

Mixed-case text is often used to indicate whispering, and is also often used for text that needs to be set apart, such as comments by an off-screen announcer (voice-over), or sound effects.

Caption decoders and televisions were not required by law to support lowercase letters at all until just a few years ago. There are, therefore, some televisions that will change mixed-case text to all uppercase.
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取自；

http://www.robson.org/capfaq/look.html