Y Combinator president Sam Altman is stepping down
techcrunch.com
techcrunch.com
To be able to keep a pulse and their finger on some of the longest tails in the marketplace (sentiment here), without that being obvious.
To a much more extreme extent, the people behind moderation on reddit.
(Not that it's a realistic suggestion for Y Combinator to move there).
Given that a key objective was to be closer to their contacts, Detroit would make little sense.
Detroit only makes sense if you like drinking or sports; there is nothing else to do there.