So something I've noticed in the fantasy genre that I love is that my 2 favorite authors (Sanderson and Rothfuss) don't use the traditional fantasy medieval setting (that I love) of castles, knights, feudalism etc. Now there are plenty of great authors that do (GRRMartin comes to mind as one that does it right), BUT the truth is, a good story eclipses all minor details like setting. An example I always give is that Patrick Rothfuss could write about brushing your teeth and it would make a fascinating read, and Sanderson would make an intriguing plot with amazing characterization throughout the dental hygiene experience. But I digress.
My question (If Brandon would be so kind as to show up, and if not, if anyone has any insight) is why; why doesn't the cosmere have any traditional medieval fantasy settings? Mistborn has keeps, but the society is not the traditional technology and setting of the medieval time period, nor do any of the other worlds given us.
There are both in-world reasons and writing reasons.
The writing reasons are obvious. I grew up on a steady diet of fantasy in a faux-medieval setting. I felt that some of these stories were really good, and enjoyed them--but at the same time, I felt the genre had been there and done that. In some ways, GRRM doing fantasy with the eye of a true medievalist provided a capstone to this era of fantasy.
When I sat down to write, didn't want to write what I was tired of reading. Dragonsteel (which never got published) was bronze age, White Sand was industrial, and Elantris was (kind of) Renaissance. (As you noticed, Mistborn is somewhere around 1820's. I modeled a lot of the society around the fascinating culture/industry of canals as shipping lanes that happened in England right before railroads took over.)
The other big reason, writing wise, is that I feel some of the magics that I enjoy dealing with in my settings need a certain near-industrial mindset to be interesting. The stories I want to tell are about people applying scientific principles to magic--and about the commodification and the economics of magic. Those are early-modern era stories.
The in-world reasoning I have is that on some of these planets, those eras existed--but the books are taking place when the stories of the worlds start smashing into one another. In addition, however, the Shards have an influence on this, because of things they saw happen on their own home planet.