The reasons we went with one day events is work/travel conflicts, expenses, CD proficiency, and participation.

We have some folks from the East make the trip and it's bad enough driving that far, but add two days worth of events in the sun to two days of driving equals some tired folks.
Also many local participants can only get one day off during the weekend. So with at least 4 rounds are flown on Saturday they opt for Saturday off and work Sunday.

The two day events are more costly and time consuming for everyone. With the second day comes an expense to the club to provide lunch for the extra day which is a load on the CD and it requires the out of town people to add an additional stay at the hotel and other expenses.

How well an event is organized or the experience of the CD plays a factor. I think with a well organized event you can successfully get in 5 rounds with 25 pilots in one day. I had about 18 contestants and was able to get at least the B-Novice finished with round 5 by 5 pm on Saturday and that's with holding the drawing that Saturday because several couldn't attend both days for work and travel reasons and I was running a single flight line in 97 degree heat.

Finally, the participation dictates the decision too. Naturally if you have 30 attendees then you'll need two days or you're really proficient and everything flows smoothly.

Tim