6  
Too much spam and low quality posts. If these were to become problems on other sites, they will also block guest posts. – Oded Jun 18 '14 at 14:37
2  
Since this is about multiple sites in the Stack Exchange network, shouldn't this be on Meta Stack Exchange? – Fish Below the Ice Jun 18 '14 at 15:28
up vote 8 down vote accepted

Stack Overflow had a problem with bad questions being asked by unregistered users, so registration was required for people to ask questions there. This was expanded to other sites that had enough volume and enough bad hit-and-run askers to justify it.

Other sites haven't had as bad a problem with this, so they're a little more open with the question-asking account requirements.

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