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It's not better...
Have a little read about line length
http://www.smashingmagazine.com/2014/09/balancing-line-length-font-size-responsive-web-design/
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But the real answer to your question is: why not?
Because it was tested in a lot of devices, proven to be the best width for readability during testing, and to justify it you're offering a sample of 1 wholly based on your subjective view with an argument that is roughly "Why not!" which is about as weak an argument that could be made for a change that would affect 300 web sites and 250,000 monthly visitors.
Why?