I can see the reasoning behind the need of 300 reputation before one can create new tags, but why do we need 2500 reputation just to suggest a tag synonym? Why is that action so much more important than the ability to create a whole new tag?
I think I now know the answer. Creating a new tag is okay with some reputation, since if it turns out to be a not-so-good tag which nobody is really using, it simply gets automatically deleted by the system in six months. No big harm done.
•new tags will be automatically culled and removed from the system if they are not used by at least 1 other question in a 6 month period
Suggesting a synonym is (if the suggestion passes) more severe action, as it effectively removes the synonym-tag from the system. Yes, the old tag is still there, but all attempts to use it will turn up the approved synonym instead.
Any tags that match active synonyms will be automatically and silently changed from their original as-entered form to the tag that the synonym points to.
That's why synonym creation is needing higher reputation than creating a new tag.