You're not allowed to flex on a contaminated runway because the FAA guidance (AC25-13) expressly prohibits this.
The same AC also addresses derates and imposes no such limitation for derate.
I believe the logic behind this is that aircraft performance is inherently less precise on a contaminated runway. There are usually only a few derate options available, if any, and so it is practically impossible to optimise the derate for the runway conditions. thus there is normally some margin on any given runway. Flex, however, is infinitely variable and so in principle zero margin can be achieved on every flex takeoff. This is not considered to be an appropriate margin combined with the uncertainty of contaminated runways.