Women are socialized to compete with each other, socialized to be dependent, socialized to be always feminine, lovely, non-threatening, especially in cults.
So, why not do the "female" thing despite the Apostle Paul's admonition against it - get gussied up - before such a social event, which is for many JWs, the only socializing they do.
Honestly, it's the only thing about being a JW that I miss and have great ambivalance about as well - always getting dressed up, making up my face, looking my best. After all, being a Jehovah's Witness is all about double standards, particularly for women. If you don't keep yourself up, you get admonished for it; if you are overly preoccupied with it, you get admonished for it; and/or just gossipped about or marked or shunned.
As a teen getting prettified was what I wanted to do anyway; as a mature (?) woman, it is something I both eschew and envy - making myself feel beautiful and desirable publically, making myself a center of attention (positive or negative), invoking the jealousy of others, and consequently, the counsel not to be "overly concerned with the adornment of the hair", etc.
It's pure socialization and double standards. Jehovah was not the reason for it at all! He's just the social proxy.