When creating a tag wiki, you do not need any reputation or experience with the tag. My interpretation is that if every user who wanted to contribute needed to be experienced, no one would suggest wikis. To make sure they're appropriate, wikis are peer-reviewed, of course, but generally, if you understand what a phrase means, it's okay to suggest a wiki.
Meanwhile, to suggest a synonym, you need at least a score of five in the tag:
Users with more than 2500 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms. Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2.
This may require less experience than creating a wiki - a whole description of the subject and its applications - and it's peer reviewed anyway - and yet you need reputation in each tag you want to suggest a synonym for.
I'm arguing that it deters people who could potentially be constructive - who wants to go out of the way to answer an old, obscure question and hope for rep just to improve a tag? - and clearly, reputation is not an issue, because we don't need rep to make wikis and they turn out absolutely fine.
Why do we need reputation to suggest synonyms when they're peer-reviewed, and wikis don't need reputation?
If there is no reason, is there a way to change the system?