You get a 1000 SC on your first day of LC. If that's a 1000-2200 SC, then they can't use you until 1400 the second day (18 hours after 2200). So day 2 you get 1400-0200 SC. Now on day 3 you can't get SC until 2000, and so on. A 6 hour SC period allows them to keep assigning the same SC start time multiple days in a row and they don't "lose" a day or more of availbility as your 18 hour leash keeps moving backwards.

But also another reason they went from 12, then to 9, and now to 6 (other than the 2359 SC period which I think is still 8-9 hours), is that they found that the futher into a long SC period you got the less usable you were due to the RAP+FDP limit. I would assume that fatigue calls were also higher when they assigned a report at the tail end of a long SC period that went close to the max. Also remember that the RAP+FDP limit is a hard limit and cannot be exceeded unless you're already in the air on your last leg.

At this point the main reason for SC isn't as much for the "hey here's a trip reporting right now when can you get here" but more for stuff that pops up 10-18 hours from reoprt that can't be covered by a LC pilot. Over the last year only 1 of my SC rotation assignments wasn't on my schedule late the night before. I've had just one close-in assignment. So scheduing doesn't need a SC sitting available for 9+ hours as long as they can layer in several 6 hour windows. Those people have greater usability and scheduling can keep rolling their SC period day to day until a rotation gets assigned.