Two reasons :

1. Since our coup and the strong anti-foreign developments of the last few months we don't want anyone being arrested by the local gendarme. 
2. The bus supplier says that even with 12 (10 team members, guide and driver) in a bus they are a bit over the legal limit, but definitely they should not have more.

If you feel you don't want the extra bus then there will be tuk-tuks. I think it is about Bht 50 one way. The main thing is though - don't carry more than 10 team members in the bus.

Please contact your local Sixes coordinator if you will need an extra bus.