Not to mention, in the '60s it was more socially acceptable for a woman to be called 'girl' as it wasn't seen as some sort of condescending term.
It was just 'hip' and it helped sell the comics to the audience of the '60s, which was predominantly kids and teens.
And calling her a 'woman' would likely remove her from being relatable to those younger readers. They'd probably view her as a mom or a teacher instead of someone on their level.
Although, today, 'Woman' would be the better sale, as the readers of today are older.
See also Hawkgirl/Hawkwoman and Invisible Girl/Invisible Woman.