Bowling captains have to be pretty special. Bowlers usually stand in the outfield, which removes them from the action. As captain, the bowler should be able to field closer in, or even in the slips. The bowler-captain has to know his place in the team and when to take himself off or put himself on. He has to understand that he might not be the best nightwatchman. Finally, he needs to be fit to play each match, he can not be a bowler who gets injured every ten matches.
Batsmen are far less injury prone, much more suited to agile slips fielding and are usually have enough perspective to effectively manage the bowling attack.