Post History

Since the tradition doesn't have any enforcement anywhere, we can't necessarily pinpoint a single reason, but it probably has a lot to do with the available technology of the early 1970s when Chamberlain and Boyce put together the original SEQUEL.  Specifically, punch cards and teletype machines generally didn't support multiple cases of character, even when the computer itself might.

However, COBOL notably follows many of the same conventions, and SQL takes a lot of cues from COBOL.  Speaking of which, [COBOL's metalanguage](https://en.wikipedia.org/wiki/COBOL#Metalanguage) (used to describe the language) might also have some influence, where it uses all-uppercase to identify reserved words.

I should also mention that FORTRAN has a similar tradition, but while it has a similar age to COBOL, we haven't generally heard about programmers developing with FORTRAN and SQL.