My thoughts as to the reason some regionals are two days and some are one is the cost of renting the venue, cost of a hotel room (and if they can get a discount for the rooms) and what there is to do in the area. I know at my old club in the bay area of California it was really hard to find a venue that didn't break the bank for the club just for our one day contest. I realize that California is not the normal for costs but still that's the main issue for clubs. Also if there is nothing to do outside hanging around a contest for 2 days I would just go for 1 day. Yes, you can say there are seminars but still if none interest me then I won't go for 2 days. There is also the issue of getting enough volunteers to run a 2 day contest. I've only been to one contest in the area I currently live in but still at that one they were still having trouble finding enough people to judge the contest. I joined a team so we had 3 people but at times had only 2 because of one person being entered in a category we had to judge.
Just my two cents.
Eric