The reason is peer review.
Tag descriptions don't get bumped to the
main page and nobody gets to see 'em.
ALL content is supposed to be peer-reviewed, that s why we don't have a minor-edit feature. First off, 1.5k is easy, and you don't want a user to put random crap/spam in a tag excerpt. How would you feel if, while typing in the tag box, a suggestion with a rather offensive excerpt came up? You'd think SO itself had put it there, it has no indication that it's entered by a lowly user (unlike posts, where if you see something wrong, it's obvious that it's not SO's fault but that of the user). Well, a newbie would feel that.
I think that even 5k users have to go through approval for tag descriptions. They can't approve their own edits.
(If you have a high score on the tag, you can). This is due to the same reason as above--5k ain't too hard to get, and tagwikis can lead to scary situations.
On the other hand, tags are shown on the post, and someone will notice if you create a bad tag.
When you think about it, almost all such restrictions are due to peer review.
Tagwikis get reviewed pretty quickly, so you really don't have to worry.