I believe it has a lot to do with getting the right cattle in the right homes.
Producers or traders (whichever is the case) that really want to make a career out of the club calf deal really need results. They may even be willing to take a little less money to get their cattle into good homes.
I believe another factor is cost. I don't know the exact number, but these online sites will charge a flat rate to "list" your cattle and allow you to use their site like a bid board. The cost is significantly less than the percentage they charge if they are handling the entire sale. So often, I talk to sellers that are disappointed when their sale is over because every buyer would have bought from them anyway. The idea is the the online sale format will bring them customers that they otherwise would have not had. I think that is true on the $2,500 to $3,500 calves but not so much on the higher priced cattle.
I don't personally like the phone bid off deal. I have never bought one like that. I have bid on a lot of them, but never ended up buying.