"Why millenials are leaving the church" has a number of problems not the least of which is that is phrased as a statement rather than a question. Rachel Held Evans does make some decent points in her CNN Belief blog post but ultimately fails to deliver on what she seems to think are decisive insights. Like many other theists who have wonder why more "millenials" are leaving organized religion in larger numbers she makes no attempt at setting aside her own biases. She stops short thinking that once she has found a few possible answers that those must be the answers. Not once does it occur to her that a percentage of this demographic may have reached the conclusion that religion just does not make sense. Personally, I have met a few who were disturbed by "Christianity to be too political, too exclusive, old-fashioned, unconcerned with social justice and hostile to lesbian, gay, bisexual and transgender people." However, that was merely a starting point for some. After thinking about it they found it was hollow and just wasn't worth thinking about anymore.
Evans also at various points slips into contradictions and superficiality. Not quite halfway through her short post she complains, "Invariably, after I’ve finished my presentation and opened the floor to questions, a pastor raises his hand and says, 'So what you’re saying is we need hipper worship bands. …'” This would have been a valid point. The idea that some clergy just can't grasp deeper issues and problems is not that unknown but Evans displays her own superficial understanding a little later on, "Many of us, myself included, are finding ourselves increasingly drawn to high church traditions – Catholicism, Eastern Orthodoxy, the Episcopal Church, etc. – precisely because the ancient forms of liturgy seem so unpretentious, so unconcerned with being 'cool,' and we find that refreshingly authentic."
"Cool"?! I have yet to come across any atheist who left religion behind because it was "cool." I'm not even sure what that means in the given context. Usually the term is used to denote something that is popular or fashionable. Is being part of a distrust demonized minority really considered cool? By who? Even though there have been reports in our numbers rising it is rather misleading. To some degree it has to do with more being willing to self-identify. Our numbers are still well below 10% when you limit to specifically "atheist." When the term "none" are applied it does rise but that includes many who actually are theists, all be it devoid of a specific brand of religion. I'd also note that there is no objective basis for concluding what she does about church traditions. You can just as easily argue that they are by their nature arrogant and repressive rather than "unpretentious."
In the end there are no one-size-fits-all answers for such a broad question. It gets even messier when you look at more than a narrow demographic like "millenial." People of all ages and backgrounds leave organized religion all over the world. The reasons are as numerous as the individuals involved. And, there is, of course, a related question theist seem to avoid completely. Why be concerned with others leaving religion in the first place? It's as if they can't sustain their own beliefs unless they are shared.