Moral rules do not exist by themselves. They only exist if most people believe in them, and a large number of people are prepared to go to some trouble to enforce those rules. Politics only works if it can ultimately be translated into direct action. Circumstances can change for a number of reasons. People change their minds about what is important.
New technology can increase or decrease the effectiveness of different types of direct action.
A moral rule can disappear just because it is unenforceable. In western societies we tend to think of politics in the context of western democracy. But politics is relevant even in undemocratic societies, as long as gaining the support of a group has some relevance to resolving a conflict. In some cases political action in a democracy can result in the destruction of that same democracy. For example someone may convince a substantial minority that it is worth going to considerable trouble to wrest power from the majority that support the existing system.