@mwichary Table 3 on page 360 of "The SGML Handbook" by Charles F. Goldfarb has a table of the _standard_ assignments for #SGML delimiters which is configurable in the SYNTAX DELIM part of the declaration. STAGO (<), ETAGO (</), MDO (<!), PIO (<?) are closed with TAGC (>), MDC (>), and PIC (>). Curly braces, part of ASCII and Unicode 2.0, are not found.
So your search is for the authorship of the #standard for STAGO, ETAGO, and TAGC. Legacy systems still in use in 1980 used a wide variety of character sets. It is possible that a legacy 6-bit character set was used as a constraint as the bias to all-upper case and lack of curly brackets is consistent with that theory.
As you want history, not speculation, I will point out that #Goldfarb wanted the system to be visually distinct because this format (like GML) was designed for humans to be able to parse and this concern would have been common. But SGML was designed and adopted by a committee building off GML and committees have documents. Good luck!