As a "millenial" (I thought we were called "Gen Y" and "Millenials" were those born after 2000, but whatever) I think it is because we grew up so simulated with technology and religious services are the complete opposite (no engagement, no benefit, no interaction, etc.). Plus some call us the "ADHD Generation" so maybe that has to do with it as well.
I know lots of millenials are doing the bare minimum as JWs and I know of only 2 that are MS out of dozens. The WT's gonna have a serious problem within the next 5-10 years when the average elder body is 70+, which its nearing in lots of cong. now in the west.
Wikipedia also has a short section about religion on the Millenials article. I find generational-topics (Baby Boomers, Gen X, Millenials, etc.) very interesting because you do see some characteristics specific to each group.