Probably because people would use it too much
I was using it alot and it appears I cannot anymore even while still on perk status (been several days like that now)
So, probably don't want it over used.
(there was no warning or messages like the one you get if you post another site)
What wrong with using it too much...this is a trade forum. Why have misinterpreted trades that could be avoided with a simple screenshot. Obviously, there's support for screenshots as you can use a third party website to link it. To me, it seems like an easy fix to help make trading easier.