Prior to July 2015, site graduation was basically decided subjectively by the SE team, based on the site's Area 51 statistics and the capacity of the design team. From what I recall, it was basically a queue-based system, with a site's "position" in the queue being basically decided by the number of "Excellent" Area 51 statistics.
However, in July 2015, the criteria changed: now the main, essential (essentially sole) criterion is that the site must consistently have 10 questions per day on average: sites that fall short of this will still be considered beta sites.
Why was this chosen as the threshold number, and why was this made the essentially sole criterion, rather than considering other statistics? In particular, why are sites with excellent answer and view rates, but with lower questions per day, considered "worse" on that basis alone? Under the old system, those sites with "Excellent" statistics otherwise would graduate.
In particular, I'm looking for statistics that make it clear that sites with fewer than 10 questions per day perform significantly worse (in terms of post quality, moderation ability, etc.) than those with more, and that there is no significant difference when considering other Area 51 statistics; basically, statistics that clearly prove that questions per day is the sole decider of a site's health and that 10 is the right number to set the threshold at.
Update: On August 1, 2019, the 29 sites that had been actively sustaining public beta status for at least seven years launched despite not having at least 10 questions per day. But this was effectively an exception, not a general rule (as evidenced by the fact that sites that became more than seven years old later on didn't proceed out of beta), so my same question still applies: why was the essentially sole criterion for being able to graduate/launch specifically chosen as 10 questions per day, without considering other statistics regarding a site?