My Thoughts on why the game Performed like that in the launch window :
1- ) The Time of Release : In the games that gets released in 9 , 10 ,11 ( The month Before TEW2 , The Month After and the Current month ) , If you are given the choice to pick up between Shadow of War , AC Origins , Call of Duty WWII , Southpark , Mario , Pokemon , Wolfenstion and The Evil Within 2 , Then I guess TEW2 won't be the first choice for a lot of people , While TEW1 in 2014 competed only with COD AW and AC Unity and Far Cry 4 in the same Window
2-) No Demo : One of the biggest if not the biggest complain that Gamers had with the Original game was its performance on every platform released , So to redeem themselves , Bethesda should had released a demo for TEW2 in all platforms as a benchmark to show that this time everything will be smooth , but instead they left customers hesitated whether to buy it nor not fearing that they will get a similar bad experience like the last one
3-) Marketing : TEW1 had a propaganda of " Return of Survival horror Godfather" , " Return of Mikami " , " Resident Evil 4 True Sequel " , " The ideal game in Mikami's mind " and so on and as we all know , Name sells games While in TEW2 not only we didn't get a Proper marketing , but they stick with the failed RE7 marketing method of letting Famous Streamers and Youtubers do the work and Over-dependence on Word of mouth and we all know this method didn't return good results with RE7
Other than those , I don't see a MAJOR point that made the Sequel perform way less than the Original sales wise , I myself has some small complains about the Art style and the Story but they are minor ones compared to the above ones and it shouldn't had affected sales to that degree if not at all