To protect Sid. From the very beginning Sid has been aggressive and Salman ne bahut pyar see samjhaya bhi, that it works against him. But Sid simply can't control his aggression n so BB is going out of it's way to not give aggressive tasks which involve Sid n whenever BB gave such tasks, Sid ended up being very aggressive n people, especially girls ended up getting hurt.
So in the rakshas task Sid wasn't made the rakshas as it was bound to be aggressive. In the last to last captaincy, it would have seemed Sid would win easily in the frame task, but that was not to be n Sid became physical. In the present captaincy task, when all d contenders were boys n physical task could have been given, BB did voting n the outcome was obvious. Also BB didn't dare make Shukla sanchalak, though he was the captain, coz sanchalak is the most thankless job and no matter what one does, theres always scope for unfairness coz the decision is made relatively when the task is going on n some audience will find it right and others wrong. So BB was simply protecting Sid from negative criticism that is bound to happen.