Are you referring to the forum karma? If so:
The issue with tracking forum karma is how would you document it? By percentage or by karma issued? You have to remember that your overall karma rating is the result of positive minus negative. If the percentage was used, the top of the list would be filled by bots with one or two posts that users mistakenly thought were genuine people.
The karma percentage takes a while (up to a few hours even) to update on user pages whenever you receive an upvote or downvote. Imagine a whole list requiring consistent updates of this manner. It would throttle the websites resources just calculating a single page.