JMO, but I think much of it is about the perception of luxury, or "specialness". If it's French, it must be ooh la la, right? And Aleppo and Savon de Marseille have been around so long, they're legendary. I'm sure there are wonderful French milled soaps, and I've never tried Aleppo or Savon de Marseille, but I'm sure they have their own great qualities, but I think something else may play a part in their popularity.
Early on in my soapmaking days, I read soap blogs and looked at soapmaker's websites and was dazzled by the "superstars" of handmade soapmaking......the ones whose name every soapmaker knew, the ones who sold lots and lots of soap. How amazing must their soaps be, I wondered, for them to actually be able to make a living out of making soap? I imagined soap bars that produced mountains of big, frothy bubbles, and would make my skin feel like a baby, whose fragrance was like something that had fallen from heaven. So I decided to order some, so that I could have this experience. And you know what? It was nice soap....but it wasn't nicer than my own. I was really quite surprised. How was this possible? Shouldn't I be hearing the angels sing when I used their soaps?
I gave it a lot of thought, and realized that these superstars were superstars at marketing, too. They had beautifully designed websites, gorgeous photographs of their products, beautiful packaging, well-written descriptions, strong branding, and they knew how to use social media to their advantage. Yes, their soap was nice, but I wasn't hearing any angels singing. That's when I fully realized just how important marketing is. If someone sells a lot of soap, it must be really good stuff, right? Well, maybe yes, and maybe no. Look at Dove. They're marketing is great at convincing people that their "soap" is special.
So once the perception is out there, that a certain product is special or luxurious, it gets rolling, and is kind of hard to stop. Not saying that either French milled, or Aleppo soaps aren't good, just that they have a certain "aura" about them that may or may not have anything to do with their actual qualities...... they've been around for so long, that they must be special, extraordinary even, right? It has a lot to do with perception.