"You are good women, not bad!"
Translation:אתן נשים טובות, לא רעות!
It's just the way the word works, and like many exceptions to rules, there isn't a logical basis for it. Singular אשה (it is typically written this way, even without vowels, and they should fix that they use אישה because they're teaching people bad spelling) and plural נשים.
There is a form נשות - it means "women of" - and is the form that נשים takes in two-noun construction, like in the name of the organization נשות הכותל, Women of the Wall. So if you use נשות when you mean נשים, native speakers will be listening for the second word in your construction, and depending on the context, you may be understood or you may be creating confusion.