Archaeology "Drop rates"

Is there a reason old Archaeology drop rates for certain rares have remained the same? I have been farming for a Zin’Rokh for the past week, every day. I really don’t know how many solves I’m up to…but it’s a lot. We can go back and solo past raids for transmog gear, but we can’t ease up the drop rates for past profession drops for transmog as well? Honest question.
Thank you