Only theory would be that bad weather up north might prevent that many games from getting played. To play 3 game sets you'd have only play on weekends. Right now softball does play weekday games so conference play isn't as long as baseball.
I would also guess that they prefer playing in all the tournaments before conference play starts.
You are very correct from what I understand. The NCAA recently put a limit on the amount of games you can play per season. So the teams in the BIG 12 like to play more teams around the nation in tourneys rather than a 3 game series. The SEC, Pac-10, Southland, and a few others all play 3 game series, but usually only 2 tourneys a year. I think they do 2 game series also so they won't miss as many classes.