One reason that feminine garments could appear to have evolved more slowly (aside from, you know, them evolving more slowly ;) is that there really are very few extant gowns of any kind prior to about 1600 CE. Much of our prior knowledge is from artistic sources, and a portion of what you see in almost any costuming book is fantasy.

So the artistic sources for our period, such as they are, tend to be either statues/effigies or illuminations. Illuminations can't always be trusted for details of women's costume, for the simple reason that if they were done by monks, there was a very good chance they hadn't seen many women's dresses very closely in a long time. (If the illuminations had been done by tailors, or by women creating their own clothes, the record would probably be better. Funny how things don't work out that way....) So the other source, and one which IIRC has been used quite a bit for early medieval clothing like the gowns in the pictures, is statuary. Usually it's ecclesiatical statuary depicting donors who funded the church where the piece is located, so what's most available is a record of what a handful of very wealthy women wore or at least what the sculptors depicted them as wearing (as often as not, the statuary was created posthumously, maybe even by sculptors who'd never seen the lady in question). Effigies, when available, are very helpful.

Common wisdom on the "dark ages" pretty much has everyone in some form of T-tunic, often very adapted for the women. Skirts on long tunics start getting fuller when you start cutting them up the side for ease of movement, then adding panels for modesty. Costumes of noblewomen, especially in more developed communities, will become more lavish in some way, eventually including "pointless" (pure-fashion) elements that are marks of status or belonging or etc.

Fads and trends may exist where people can afford to use them, but these things are so ephemeral that we're unlikely to know about them unless someone happened to mention them somewhere (like, "The Empress's ladies have taken to wearing one crimson ribbon in the left braid each Friday"). To compare - we know a lot about the fads and fashions of Heian Japan, but only because there were talented female writers working then who had an interest in those things, and their works were preserved. If we had the diaries of five or six ladies of the court of Alfred the Great, or William Rufus, and at least one of those ladies was "fashionable" - we'd know. But it's just lost knowledge.(I'll even hazard a statement that it's not really vitally important knowledge, either. It's just that we'd like to know it, it would satisfy our curiosity, maybe help us participate in reenactments if that's our thing.)

To sum up, what I'm trying to say is: any illustration of Anglo-Saxon or Norman fashion is going to be based at least partially conjecture, if hopefully educated conjecture. Furthermore, the idea of a slow development of fashion is both true and false: basic shapes often take a long time to change, but smaller details - not so costly or difficult to alter - change all the time. I don't know if I pull out of period too much when I'm writing here, but I think a modern analogy is how there are "fashion classics" that women are advised to purchase - black cashmere sweaters, very nice pants - and a woman who does so, and wears them, can "always look fashionable" by changing her jewelry, handbag, hairstyle. The T-tunic-based gown was the "fashion classic" for Anglo-Saxon women, the theme on which to elaborate. I'm sure they weren't always or even usually thinking about fashion, but (to put it mildly) I can't think of a society where it hasn't at least cropped up from time to time.