Though their initial mutations were the result of long term nuclear war, it's actually an intense culture of propaganda and strictly enforced conformity policies that have kept the Daleks in their current state. Every Dalek undergoes surgery just after birth; not to mitigate the effects of irradiation, but to rid them of whatever is legally considered "imperfections to the Dalek form" (usually throwbacks to a humanoid body plan, such as teeth and hands) and physically ready them for their casing. Additional surgeries must occur throughout a Dalek's life to prevent them from becoming "rooted" - a painful, often fatal scenario in which the Dalek overgrows their own shell and cannot be safely removed. Not only does their homogeneity serve to reinforce their fervent belief in genetic superiority, but the resulting chronic health issues prevent insurrection; a Dalek's fear of losing access to regular healthcare colors much of their judgement, and most attempts at dissent end as soon as their painkillers run dry.
