I do not know the specific answer. But could venture some theories why:
From
http://www.ncaa.com/news/softball/article/2015-05-01/2015-division-i-softball-conference-championships-automaticConferences without conference championships: Big 12, Big West, Mountain West, Pac-12, West Coast
One could infer that all are from west coast, minus Big 12. West coast schools tend to be on the Quarters system vs Semester system for the rest of the country. For schools on semesters they are finishing finals as their conference tournaments start. For schools in quarter system they would just be heading into their busy time
Some argue that in strong conferences, league championships only serve to guarantee losses against strong conference opponents. Those losses coming right at the time the committee is deciding seeding. Ultimately hurting the overall number of teams a conference may get into the tournament.
http://robocoach.websitetoolbox.com/post/hypothetical-pac10-tournament-4691130 Conferences that do not play conference championship, often have a round robin, so if you look at their schedule they are often pushing hard through conference weekend to get all of their games on the schedule. They also can declare a league champion, since every team has played every team.
This year Pitt winning ACC definitely put some of the bubble ACC teams, Virginia Tech and Louisville in a bit of a bind. Committee ended up placing 7 teams from ACC in the tournament. Some might question that. Since that is the same number as Pac 12, it may be argued, at least for ACC that how their tournament went ended up getting an extra team into NCAA.