Best I can tell it's a common courtesy between the various folks that hold events on the salt.
Each one puts in an enormous amount of work to prep the salt for their event, and it could, at least on some level, create problems if some other group came in and screwed up all their work.
I agree if you knew for sure that you're running the last event of the year, it shouldn't make any difference, but otherwise not stepping on each others toe makes sense to me.
I'm just glad we got to run this year, so I'm not that hard to please, just point to where you wont me to run and I'm happy.
Rouse