Comes to down to quite a couple of reasons.
- As Rik said, its hard to think about your field, bowling changes, field in the ring etc,. while bowling.
- If your team has a bowler as captain, he/she needs to keep feelings of their own bowling out of their mind, and bowl whoever is best suited to the situation. Sometime they can overbowl themselves, or equally as bad (although rarer) you will find captains who underbowl themselves for fear of looking like someone who rates themselves highly. Few bwling captains give themselves the right amount of overs.