Why do you need so much reputation in order to propose Tag synonyms?
Because we want users to realize the usage of tags and how they work before they can propose tags synonymous.
Anyone with some reputation can edit, remove, fix posts with some reviewing associated. It is ok and part of the site that the community is responsible for them.
Tag synonymous on the other hand gets registered in the site and will affect the inner works of the site, you should at least have spent some time understanding how the site works before the site allows you to do so.
Tag synonymous requires votes to pass, I would hate to be reverting synonymous voted by a group of low reputation users or to have the suggestion list filled with noise, even so, the reputation required is not has high as you think and as I wanted.
Another point of the way we work is that even a user with a low reputation can suggest a tag synonymous: you can do so via a meta post with that proposition. We can then further discuss, agree or disagree and move to action if in favor and create the tag synonymous.
Other than that it requires that users with some reputation and that participate on the site agree and vote for them.