Everything I've run across for years now indicates it's a combination of Dunn's law and annoyance with fees driving the shift.
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"Fees matter. That is the overarching sentiment that emerged from an online Wall Street Journal survey asking readers about their investment style and satisfaction as part of the Journal’s series looking at the rise of passive, or index-based, investing."
http://www.wsj.com/articles/readers-react-low-fees-are-drivi...
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"Lower fees, as opposed to better performance, may be the big reason more investors are switching from active to passive. Morningstar found the investors in active funds improved their odds of beating passive funds by favoring funds with lower-than-average expense ratios."
"It's not so much about active versus passive as it is about fees," Morningstar's Johnson said.
http://www.cnbc.com/2016/08/29/investors-say-forget-it-to-ac...
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And this has been going on for ten years, which implies a sustained shift of understanding rather than a blip:
http://www.wsj.com/graphics/passive-investing-five-charts/