They announced the end date of the season about a month ago.
All you had to do was ask. 
In your defense I don’t think that they should have immediately removed the unranked conquest gear from vendors, because I don’t think that was really mentioned anywhere.
Maybe the purple sets were “seasonal” in the same way vicious mounts are in that you have to earn them now if you don’t want to wait until next expansion when they’re added to vendors.
And they’re probably removed from vendors so people don’t accidentally buy last seasons gear in the current expansion, etc.