The folks responsible for the sexy costumes, Roddenberry and Theiss, died in 1991 and 1992, respectively.

See also: https://tvtropes.org/pmwiki/pmwiki.php/Main/TheissTitillationTheory

The sexiness of an outfit is directly proportional to the perceived possibility that a vital piece of it might fall off.

This basic theory underwrites Stripperiffic clothing, Impossibly Cool Clothes, and pretty much anything else you stick characters into: what makes clothing sexy is the potential for a catastrophic Wardrobe Malfunction. The Trope Namer is William Ware Theiss, costume designer on Star Trek: The Original Series, who first codified the concept.

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Though Theiss was a costume designer, according to Inside Star Trek: The Real Story by Herb Solow and Robert Justman, most of the costumes — following this theory — were actually somewhat more modest before being "improved" by Gene Roddenberry.