There wasn’t any version of the game available for me to test against until recently, and we had to spend a lot of time updating Shadowlands for the new season right around when the WotLK PTR became available. Only one of me, and only so much time in the day!
Also, we generally have not worried much about pre-patch periods for any version of WoW. They are transitional, not balanced well, and generally have little to no theory work done for them by the community. We focus on max-level PvE content for each expansion, thus we’re working on level 80 stuff for WotLK.