I know that Activision knows that the general opinion is that MW19 was the last genuinely great COD, and I get the feeling that they're going to be milking that perception for the upcoming release of MW4; this sale is probably the first part of that. Drive up player numbers, get people wanting a newer, better version of what they're playing and what they remember.
I fully expect the advertising campaign for MW4 to be a mix of nostalgia baiting along with the good ol' COD bait and switch wherein a gritty, grounded experience is advertised, but in reality we get another first-person Fortnite. They know they have to follow up the horror show that is the MWIII campaign, and they're going to make it look just as impactful, engrossing and enjoyable as MW19's.