In my opinion, this is not a drastic difference.
When you post a normal answer, you put some effort in it. No one can make an edit to it to add more information because it would deviate from the original intent of the post and wouldn't preserve your goals. If the effort you put is small, then your answer would be downvoted and you'll lose reputation. If it's big, it'll be upvoted, and you'll earn reputation. And that's fair because YOU are the author of the post and responsible for the possible reputation change.
However, posting a Community Wiki answer is like saying 'I post an answer to help the asker. I know this answer is not the best and that it can be improved, so I am going to let <2k rep users to edit it'. This way, drastic changes can be done without being rollbacked from both high-rep and low-rep users. So, if many people edit the post, wouldn't it be unfair if reputation from upvotes go to the original poster? Yes. In addition, people create CW posts to add a comment as an answer. However, since the answer isn't theirs, they renounce its ownership turning it into a Community Wiki.
Therefore, that's why you don't earn reputation from upvotes and accept votes if you are the OP of a CW post. You are rewarded, however, by earning the reputation from bounties offered.