Cytoplasm carries the organelles, which are things like mitochondria, chloroplasts, etc. They are inherited only from the mother. If there is a genetic defect in the organelles, then the plant can’t produce pollen. Therefore, Mendelian inheritance doesn’t apply. The offspring will carry the defective organelles in each generation. Sometimes, the deficiency is due to a disharmony between the organelles and the nucleus DNA, and rarely getting the right pairing can resolve the CMS. But it’s rare, and can’t be expected.
In brassicas, the CMS is due to the organelles being from a different species than the nuclear DNA. They are essentially GMO.