A super-interesting question that is also an invite for people to flog their own personal theories. People will tell you lots of reasons: increasing incomes, the development of ubiquitous media, increasing diversity, weakened religious control mechanisms, etc.

There is probably at least a little truth to each of those, although I think many of them are also effects of central causes (e.g., religious control over common people's lives declined because of increasing incomes, which increased due in part to advances in communications tech).