Well, the team's last captain, Rajon Rondo, was a bit of a disaster, so maybe they shouldn't name one. I think a case could be made that anyone of the four could be that guy, therefore, maybe Stevens could be reluctant to name any of them.
Out of the four, I think Kemba, Smart and Brown are probably the most natural leaders. Kemba might not be here past this contract, which probably rules him out. So between Smart and Brown, I am not sure which would be better.