The vote wasn't about kicking him out of the democrat caucus, just stripping him of his committee chair. Committee chairs are leadership roles within the party. When he has opposed the democrats on various issues relating to his committee, wouldn't you think he would be replaced by someone who would side with party? By taking away his prized committee he would have faced a simple choice: be relegated to nothing more than a vote in the majority, or hop the fence and join the minority party that he by and large votes against. Do you honestly think he would abandon policies he has supported for most of his career just to play a minor role in the opposition party?

Instead they take away his subcommittee in which he has sided with dems, but left him a committee that he uses to oppose the democrats. Because that makes perfect sense.