From the linked Pew survey: "Former Catholics who are now unaffiliated are much less likely than lifelong Catholics to have attended Mass regularly or to have had very strong faith as teenagers." Also, "Former Protestants who are now unaffiliated are less likely to have regularly attended worship services as a child and even less likely to have attended regularly as a teenager."

It looks like millenial Catholics and Protestants stopped caring and eventually drifted away because their parents stopped caring and drifted away. We're not talking about millenial's anger at Church teachings; we're talking about indifference across generations. It seems like everybody is trying to fight their adherents' indifference, but nobody is really succeeding.