I love how people always say "Wow, big numbers, it's a big success, you can't say it underperformed!", while completely forgetting to compare the numbers to the previous movies. If the previous movie really sold 6M, then 3M is an underwhelming performance. And I don't understand the "it's already a hit", since BR and DVD sales are a lot stronger closer to the release.
It's like saying "Wow, GTA6 sold 20 millions, it's better than any other recent games, you can't say that it underperformed!", while ignoring that the previous game made 3 or 4 times that (hypothetical scenario, of course).
Oh, that looks like a sharp drop of home video sales from TFA. And BP will outsell TLJ within the month.
I'm not saying it's even the main factor in the difference of sales, but The Last Jedi is on Netflix, has been for a long time, and The Force Awakens isn't.