Many women in this part of the world have little power until they have a daughter-in-law to help them do housework. When that daughter-in-law becomes a mother-in-law she'll expect the same thing!
Given how widely people's personalities differ, it's difficult to explain this in a general way. Some aspects of being a mother-in-law in India and Pakistan are basically different from the way things are here. A young woman who marries and joins her husband's household often has little say about how things go in her new family. Sometimes a young bride is treated like a servant - there are tasks that fall to them traditionally: cleaning wheat, grinding flour, fetching water, sweeping the courtyard. They have little say in family decisions, many of which affect them directly. Their preferences aren't solicited or considered. By the time they become mothers-in-law they're ready to wield power over their own sons' wives. Of course there are many, many individual variations to this pattern: many women are compassionate because their own lives as young wives were so difficult.