I didn't realize that moving up a level could be voluntary. I thought you won your way out, period. Not true?
Yup. Here you can stay at a level or promote yourself. The biggest reason I think is that you need to have 6 couples in the final for the result to 'count' - but many catagories have very few competitors and hence, you can win dozens of comps without getting one that 'counts' to your required three.
It actually works because of self-policing. The reason for the constraint to not move up too quickly is because you can't move down again. Thus, if you are a star in Bronze you think twice to moving up to Silver until you are competitive in that category. A lot of couples move up out of frustration from Gold to pre-championship simply because they are tired of syllabus and want to dance all out in costume.