cinnamon in bread dough
I think there is no reason except tradition. Cinnamon rolled buns just happen to be a common food in Central Europe, and they were exported to the USA. 

While I have no source to back this, I have frequently made breads where the cinnamon is kneaded into the dough. One common example is Peter Reinhart's Greek recipe from the book Bread Baker's Apprentice. The bread is enriched with honey, olive oil and a good amount of cinnamon, and it rises and bakes just like any other bread with similar hydration.