Maybe she lacks the confidence to make what she sees as a bolder ie less conservative fashion statement. Some styles of clothing, jewellery, hairstyles, make up etc take a certain amount of confidence to pull off because of the attention and comments the styles will attract from others, both positive and negative.
Or, sometimes saying "that looks good on you , but I would never wear it" could be a polite way of saying "I would never be caught dead in that ."